6 Inheriting Genes ©2006 by Kendall Hunt Publishing Company. Reprinted by permission 6 8 9 0 B U © udaix/Shutterstock.com Porphyria treatment of the future J O S H U A The porphyria gene is on a chromosome ©Giovanni Cancerni/Shutterstock ©Nikolay Litov/Shutterstock.com S M I T Hthe famous artist, was Vincent Van Gogh, believed to have been , afflicted with porphyria ©irisphoto1/Shutterstock.com Essentials Porphyria gene on chromosome separates into sperm and egg (a) Pedigree of Family with Porphyria © Kendall Hunt Publishing Company 191 ch06.indd 191 11/12/15 4:31 pm 192 Unit 2: Is it all in the Genes? Check In From reading this chapter, students will be able to: • Explain how inheritance of genes affects our health and society. • Trace the discovery of laws governing heredity. • Discuss Mendel’s experiments and the principles of genetics he derived from those experiments, using and explaining terms such as dominant, recessive, Punnett square, and codominance. • Describe the stages of meiosis, its products and its role in fertilization. • Explain and give examples of single-gene characteristics in humans. • Enumerate and explain non-Mendelian patterns of inheritance, explaining how a pedigree can be used to trace gene flow in families. • Use population genetics principles to trace gene flow in populations. Sevaluating its products’ impacts on human society. • List and describe the branches of gene technology, The Case M I T H of , the Vampire Diary ©Ellerslie/Shutterstock.com Diary entry: February 13, 2013. Last month, during our college semester abroad, we experienced J tell another person. I can’t be sure, and maybe I am something I will never crazy, but I know itO happened… and it changed my life forever. It all began when we spent a month in Europe. My mother’s family S came from what was once the Austro-Hungarian Empire. They immigrated H ago and did not speak much about their lives in the to America many years old country. The town U they came from, Sibiu, was now in Romania. Before the wars, the area was German and was called Hermannstadt; before 1918, (b) A Town in Transylvania. From A it was in the province of Transylvania. Biological Perspectives, 3rd ed My friend and I rented a small car, a Yugo, and made our way to Sibiu by BSCS from Vienna. The day 6 we left was hectic, and the sun was very bright. I did not like the bright sun; it always made my skin ache. It was just the two 8 the rest of our class, which stayed back in Vienna. of us taking a weekend away from 9 acquaintances. He was bored of the party scene in We were friends, in a way more like Vienna and wanted to immerse in0the local culture. So he decided to accompany me to my ancestor’s home town. He sure got what he wanted. B to Romania, with clouds quickly moving overhead, It was a cold night when we got UKeep in mind, I wasn’t scared at all – I had no idea of making the moon appear ominous. what was yet to come. All of a sudden, the Yugo started to sputter. The car shut down and my friend yelled, “You dummy, you forget to add gas to this thing!” I was embarrassed and really felt bad about letting him down. I knew it was the sunlight that confused me when I picked up the rented Yugo. We were tired, and there were no houses along the road. “At least it isn’t snowing,” I said meekly to try and break the cold mood between my friend and me. There was no response as we walked through the fields. There was also no road – it had ended at an open field with no sign of civilization. In the darkness, over on a hill in the distance, we spotted an old house. As we came closer, it was more like a hut, with clapboard walls and a rundown porch. I told my friend, “Let’s keep on going . . . Sibiu couldn’t be too far off.” I knew this was a lie but I had a bad feeling about the place. There was no response, and I knew my friend was bent on going to the house for gas. ch06.indd 192 11/12/15 4:31 pm Chapter 6: Inheriting Genes 193 We knocked on the door, with enough force to make it heard. After a long time with no answer, we started away. As we were leaving, an old lady opened the door. “Come in out of the cold. You must be Americans.” I told the lady that my family had come from this area a long time ago. The lady was the last of the Germans left in Transylvania. “You are one of us then!” she exclaimed. She came very close to my face, looking deep into my eyes. She remarked inappropriately, “You look like my father did when he was young.” As we sat in her parlor we explained that we needed just a bit of gas to get us to the next town. The room was creepy, but the lady was very agreeable. “I’ll get my brother, Herbie, to fetch some gas.”After she left the room, we waited and waited, but no brother. Then, my friend felt something behind the couch – it was a man lying on the ground! “I see you have met Herbie,” said the old lady. “He’s been drinking and needs his bed. Would you help him up?” This was getting to be too much, but each of us grabbed a limb to carry him. At that S moment in time, we froze, looked at each other, and knew something we dared not say – M Herbie’s skin was scarred, this man was dead. His flesh was cold, and his skin bloated. teeth were fangs, and his face appeared almost wolf-like. I He looked just like a vampire. My friend and I looked at each other but said not a word. T for one last rest. It was then We brought Herbie up to his bed and laid him down that he sat up, looked at us and thanked us. He looked atHme and said, “You look like my father!” I ran out of the house as fast as I could, maybe,15 miles to the town of Sibiu. I now know that my family was from vampires; maybe their father was my grandfather or maybe I inherited their vampire ways, somehow. But I knew one thing – I am J a vampire too. O S Check UpHSection U In the story, Herbie has a blood disorder called porphyria. It is an inherited disease, occurring in about A of his red blood cells, called hemes (which carry 1 in every 25,000 people. Enzymes that produce parts oxygen) are not formed properly. More specifically, heme groups, or substances that store oxygen in blood cells, are not formed correctly in porphyria. Without these enzymes, porphyrins (parts of hemes 6 in red blood cells) build up, causing lesions in the body. 8 Symptoms include sensitivity to light (photosensitivity); craving for blood (due to a lack of heme groups); receding and bloody gums making teeth look 9 like fangs; scabs and lesions from sun; organ damage; and rampant growth of hair in body parts 0 to appear wolf-like. Porphyria sufferers need blood transfusions to replace their deficient hemes. We cannot be sure if the college student who narrates B appear during late adolescence. However, it is the story has inherited porphyria. Its symptoms usually possible to manifest later in life. U Study porphyria to determine its genetic and/or environmental causes in more detail. How might porphyria have contributed to the myth of vampires in our society? Do you think the narrator in the story had porphyria, based on your research of the disease? Unraveling the Mystery of Inheritance Chapter 5 described the molecular players in gene transfer; in this chapter, we look at the processes underlying inheritance. We begin in a small garden monastery in the 1800s. Gregor Mendel (1822–1884), an Austrian monk who failed out of a science teaching major in college, discovered how we pass traits onto the next generation. ch06.indd 193 11/12/15 4:31 pm 194 Unit 2: Is it all in the Genes? Heredity The passing of characteristics from parent to offspring. By the mid-1800s, it was generally accepted that ova and sperm both contribute genetic information to new offspring. Most biologists believed, at the time, that inheritance from parents occurred as a blending of characteristics. In this view, traits from both parents averaged together to produce new, unique offspring. Seeking to discover if there were specific patterns in the inheritance process, Mendel devised a set of experiments using pea plants as his subject. Using pea plants to study inheritance was not original, but his approach to understanding how we inherit our traits was unique. The passing of characteristics from parent to offspring is known as heredity. Through his experiments, Mendel was able to successfully develop the basic principles of heredity. Mendel’s experiment was successful for a few reasons: 1) The garden pea plant Mendel chose was commercially grown at the time, reproduced quickly, and possessed traits easily measured by simple observation. The garden pea plant self-pollinated, meaning that egg and sperm from the same plant S would unite. The pea plant’s sexual structures were enclosed by a petal capsule, M preventing cross-pollination from other plants. Therefore, Mendel could control cross-breeding with selectI plants and not worry about accidental cross-pollination. 2) Mendel chose measurableTvariables to study; those that were clearly discernible. He selected seven pea plant traits to study because they were clearly one of two H alternatives. These seven traits included shape of seeds, color of seeds, shape of , of plant, color of flower, and position of flower. For pods, color of pods, height example, plants had either round or wrinkled peas; and either yellow or green peas. J 3) He used mathematics to measure and expose patterns in his results. Figure 6.1 O experiments. Note that the frequency of plant shows the results of Mendel’s characteristics shows distinct S proportions resulting from the crosses in each generation. H 4) Mendel’s experiment was careful, logical, and sequential. His steps were meticU Mendel credited any successful science experiment ulous and well thought out. to certain attributes, stating A in his original paper, “The value and utility of any experiment are determined by the fitness of the material to the purpose for which it is used.” © rook76/Shutterstock.com 6 8 9 0 B U Figure 6.1 Gregor Mendel is the father of genetics and was an Austrian monk who discovered the laws governing patterns of inheritance. ch06.indd 194 11/12/15 4:31 pm Chapter 6: Inheriting Genes 195 While Mendel’s findings were groundbreaking, they were unrecognized for 35 years. In 1865, Mendel reported his experiments and results in a paper presented to the Natural Historical Society in Bruenn (now Brno, Czech Republic), the Austro-Hungarian Empire. Scientists in the audience dismissed his findings. Afterward, Mendel returned to his monastery, tending to his priestly duties; he was ignored and unappreciated by the scientific community. It was not until after his death, in 1900, that biologists began to build upon Mendel’s paper. Mendel’s work eventually was recognized and discredited the blending of traits perspective. Instead, his findings showed that traits were inherited as discrete units from each parent. Mendel thus began a scientific revolution in the field of genetics. Gregor Mendel is now recognized as the father of genetics for his work on pea plants. Let’s take a closer look at the laws of heredity that Mendel formulated so long ago. Mendel’s Laws S M Law of Dominance I Let’s revisit Mendel’s work: First, he chose to crossbreed certain plants. For example, T Mendel noted that one variety of plant always produced yellow peas, while another proH duced green. He took the male anther portion of a yellow pea plant and dusted the female stigma of a green. He called these original parents the F0 generation. When he , crossed the two, all of the offspring were still yellow and none of them were green. This first cross Mendel called the F1 generation. Any trait that appeared in the F1 generation he called dominant for that characteristic. He surmised J that any dominant trait covers up Next, Mendel crossed the alternative characteristic of an organism in the F1 generation. O the organisms in the F1 generation in the same manner and their offspring were analyzed, S the F2 generation. Mendel conducted what is now termed a monohybrid cross. This is a H mating between two organisms, each having both characteristics for a particular trait – in this case both yellow and green. It is termed “mono-” because it looks at the inheritance U of only one trait. Mendel surmised that although all of the plants of the F1 generation Aable to be given to offspring. were yellow, they harbored a hidden green characteristic Mendel formed a hypothesis: the covered-up trait would reappear in the F2 generation. Indeed, he predicted correctly that some offspring 6 of all-yellow plants would be green. He was correct; the covered-up trait always reappeared in the F2 generation, 8 that a dominant trait covers up bred from parents that did not exhibit the trait. The idea another is known as the law of dominance. He called the9characteristic that is covered up the recessive trait. In his experiment, the yellow trait was 0 dominant, and the green trait was recessive. The original parents, the F0 generation, he deduced, were each pure – the Band the green parent had only yellow parent had only dominant yellow characteristics green characteristics – but that these characteristics would U pass along in a predictable manner through each generation. Law of Segregation When Mendel analyzed the F2 generation, he found that a certain proportion always appeared in his data. Note in Figure 6.2 that the F2 generation for all seven characteristics he chose had a roughly 3:1 ratio of dominant to recessive characteristics. Because the appearance and disappearance of traits occurred in constant proportions, Mendel inferred that traits must be inherited as two separate, discrete units. We now call these units alleles. Alleles are alternate forms of the same trait. For example, if a pea has a yellow or green color possible, then either a yellow or green allele is ch06.indd 195 Dominant The trait that covers up other forms of the characteristic. Monohybrid cross The mating between two organisms, each having both characteristics for a particular trait. Law of dominance The idea that a dominant trait covers up another. Recessive The trait that is covered up by a dominant trait. Alleles An alternative form of the same trait. 11/12/15 4:31 pm Unit 2: Is it all in the Genes? P generation F1 generation F2 generation Ratio Long × Short All Long 787 Long, 277 Short 2.84:1 Purple × White Flowers All Purple 705 Purple, 224 White 3.15:1 Axial × Terminal flowers All Axial 651 Axial, 207 Terminal 3.14:1 Green × yellow pods All Green 428 Green,152 Yellow 2.82:1 Smooth × constricted pods All Smooth 882 Smooth, 299 Constricted 2.95:1 Yellow × Green seeds All Yellow 6,022 Yellow, 2,001 Green 3.01:1 Round × Wrinkled seeds All Round 5,474 Round, 1,850 Wrinkled 2.96:1 Total All Dominant 14,949 Dominant, 5,010 Recessive 2.98:1 S © Kendall Hunt Publishing Company 196 Figure 6.2 Results of Mendel’sMexperiments with pea plants F1 and F2 generations. Law of segregation The hypothesis that states that there are two separate, discrete alleles that could be inherited separately. I T responsible for it. The hypothesis that there are two separate, discrete alleles that could be inherited separately is known H as Mendel’s law of segregation. We are now able to trace the, movement of alleles from parent to offspring using a Punnett square. While this square was not actually used by Mendel, it derives from Mendel’s law of segregation. A Punnett square is a diagram based on the law of segreJ gation that is used to predict the probability of inheritance of alleles between parent and offspring. Figure 6.3 uses a Punnett square to show how alleles are discretely passed on O to a new generation. The mother’s alleles appear on one side of the box and the father’s S on the other side. Each parent in the figure has two possible alleles based on their H are dominant and lower case alleles are recessive genetic make-up. Capitalized alleles in the Punnett Square. Alleles from U each parent have a 50:50 chance of segregating into an egg or sperm, eventually forming a new organism with a new genetic make-up. A • To recall from Chapter 5, an organism’s genetic make-up is known as its genotype. The expression of that 6 genotype is an organism’s phenotype. In other words, how an organism appears is its phenotype; and what comprises inside an organism’s genes is its genotype.8 9 Porphyria An inherited disease which is characterized by abnormal metabolism of the blood hemoglobin. The Punnett square gives the probability of producing an organism with a particular 0 genotype within each box. Each box of the Punnett square represents a 25% chance that B in the offspring generation. Figure 6.4 shows the an organism’s genotype will appear process of allele transfer betweenUparents to offspring in porphyria. In our story, acute intermittent porphyria (AIP) is a dominant trait, meaning that if a person has one allele for it then he or she will have the disease. Law of Independent Assortment In another set of experiments, called a dihybrid cross, Mendel mated plants tracing two different traits – pea shape and color. In a dihybrid cross both parents possess dominant and recessive characteristics for a particular trait. It traces the inheritance of two separate traits at the same time. The term “di-” is used because it looks at the ch06.indd 196 11/12/15 4:31 pm Chapter 6: Inheriting Genes YYRR x yyrr 197 YyRr x YyRr Yellow and round YYRR YR yr Green and wrinkled yyrr YR Yr yR yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr YyRr YyRr All yellow and round (YyRr) S Yyrr yyRr yyrr M F2 generation I 9/16 Yellow and round (1 YYRR, 2T YyRR, 4 YyRr, 2 YYRr) 3/16 Green H and round (1 yyRR, 2 yyRr) , 3/16 Yellow and wrinkled (1 YYrr, 2 Yyrr) 1/16 Green and wrinkled (1 yyrr) © Kendall Hunt Publishing Company F1 generation normal parent p P p P P P p p p p Parent with Porphyria 50% of offspring have the disease (a) © Kendall Hunt Publishing Company p P 6 8 9 0 B U (b) © Dundanim/Shutterstock.com J O Figure 6.3 Principle of segregation. Alleles separate into opposite ends of the cell. Alleles move independently of each other, accordingS to Mendel’s laws, with equal chances of being transmitted to offspring. Note thatHany one of the four gametes produced by parents in the figure could be transmitted U to the offspring generation. The Punnett square shows the relative probability for each gamete to give rise to its A genotype. Figure 6.4 a. A Punnett square for porphyria, a cross is shown between a parent with a dominant gene for porphyria and a normal parent for the F1 generation. The Punnett square shows that 50% of offspring will exhibit the disease. b. The disease porphyria is the likely basis for the legend of vampires. ch06.indd 197 11/12/15 4:31 pm Unit 2: Is it all in the Genes? Law of independent assortment The idea which tells that each pair of alleles is sorted independently when sperm and egg are formed. Homozygous The condition in which a pair of alleles is the same. Heterozygous The condition in which alleles a pair are different from each other. inheritance of two traits. One organism had yellow, smooth peas while the other had green, wrinkled peas. As shown in Figure 6.5, yellow and smooth are dominant traits, while green and round are recessive. When he analyzed the offspring of these crosses (the F1 generation), Mendel determined that traits were not inherited together. Instead they independently assorted as they were passed from one generation to the next. Wrinkled and round were found alongside smooth and green. All of the possible types of pea plants showed up in the F1 generation of this dihybrid cross. In fact, these organisms also showed a pattern of proportions in a 9:3:3:1 ratio, with the dominant traits occurring most frequently (Figure 6.5). All new combinations of traits appeared in the next generation, each inherited separately from each other. The idea that each pair of alleles is sorted independently when sperm and egg are formed is known as Mendel’s law of independent assortment. Two factors are inherited separately, one from a mother and one from a father. Thus, once together, they occur as either an identical pair or as a pair with different compoS nents. When a pair of alleles is the same, they are called homozygous. When both are M dominant forms, they are homozygous dominant. When both are recessive, they are homozygous recessive. When alleles in a pair are different from each other, they are I called heterozygous or hybrids for that trait. T In the porphyria case seen in our story, the disease is held on a dominant allele. H for porphyria, whether homozygous dominant or Thus, if a person possesses an allele heterozygous, he or she will get the , disease (see Figure 6.4). Only a homozygous recessive individual does not exhibit the disease. If P = the allele for porphyria and p = the allele for the normal condition, then an individual with PP, homozygous dominant or Pp, J trait. Only a homozygous recessive, pp will have a heterozygous will have the porphyria normal blood condition. O seed shape seed color round yellow wrinkled green seedcoat color S pod H shape U A 6 8 colored9 inflated 0 B U white constricted (a) pod color flower position stem length green on sides long yellow at end short © Kendall Hunt Publishing Company 198 Figure 6.5 a. traits of pea plants, with the top row dominant and the bottom row recessive; b. Punnett square for a dihybrid cross for pea color and shape in pea plants. A cross is shown between two parents with both traits. The Punnett square below shows a 9:3:3:1 ratio in offspring characteristics. ch06.indd 198 11/12/15 4:31 pm Chapter 6: Inheriting Genes 199 First-generation plants When gametes are produced via meiosis all are YR. YR YR YR Second-generation plants = sperm e. = egg yr YR yr YR 6Yr yYrr 8 yYRR Yr yyrR 9 yYRr YYrR YR yyRR 0 YYrr YYRR yyRr B YyrR YYRr Yyrr U YyRR yYrR YyRr } } } } Yellow round Yellow wrinkled Green round Green wrinkled 9 3 3 1 (b) ch06.indd 199 yr J O These plants produce four kinds Sof gametes via meiosis H U YR Yr yr yr YR Yr yR yR A yR Third-generation plants yr S M YyRr YyRr YyRr YyRr I When these zygotes grow via meiosis T into plants all body cells have YyRr genotype, therefore all produce yellow-round seeds. H , yr Figure 6.5 When gametes are produced via meiosis all are yr. When fertilization occurs, all possible combinations of gametes result in one kind of zygote. c. d. b. f. yR yr When random fertilization occurs, 16 kinds of zygotes are produced yyrr g. When these zygotes grow into plants, they produce third-generation plants with a 9:3:3:1 ratio of seed colors and shapes. © Kendall Hunt Publishing Company a. (continued) 11/12/15 4:31 pm 200 Unit 2: Is it all in the Genes? Testcross A known homozygous recessive organism is mated with a dominant organism. y y 9 0Y B Y Uy Y y ? y? unknown genotype y? yy = if a green phenotype shows up, the unknown genotype contained a recessive allele © Kendall Hunt Publishing Company Testcross How do we determine the genotype of an organism?—, it is not always obvious from its appearance. Consider a green pea plant that inherits two green recessive alleles, one from each parent. It is green in its phenotype, indicating that it inherited two green alleles. If the plant had inherited one green and one yellow allele, it would have been yellow. When a yellow plant appears, it is more difficult to know its genotype without knowing its history. A yellow pea plant has a dominant allele, but does it have a recessive that is covered up, or is the other allele dominant as well? Through using a testcross, the genotype of the yellow pea plant is explored. In a testcross, a known homozygous recessive organism, for example, a green pea plant is crossed with a yellow phenotype. The green pea plant, we know, has two green (recessive) alleles. But what is the yellow plant’s genotype? In this case, we do not whether the plant with yellow peas is homozygous dominant or heterozygous. In this testcross, a hidden recessive is most likelySto be revealed. The homozygous recessive individual (the green plant) has the best chance M of passing all its recessives to the next generation. ­Figure 6.6 shows a testcross between a yellow pea plant and a green pea plant to I determine whether or not green peas will result in their offspring. The testcross helps to T yellow pea plant. determine the true genotype of the The appearance of a recessive H in the testcross’s progeny is the only definite proof that the unknown genotype was indeed a heterozygote. In other words, if one of its off, spring is green, then alleles coming from both parents must have been green. For the new offspring to have become a homozygous recessive, one recessive allele had to come from the yellow parent plant. However, if there is no individual with a recessive trait J for pea color in the F1 generation, it may mean simply that the recessive allele may get expressed in another generation. O Perhaps that recessive allele simply did not get passed S along this time around. It is impossible to know for sure, but a testcross gives the best chance of being able to reveal theHtrue genotype of an individual with a dominant phenotype, such as the yellow plant. U are so difficult to study and/or remove from a This is also the reason recessives group. They are hidden, and onlyAchance dictates whether or not an allele will become expressed. Many times recessive traits are deleterious, or cause harm to an organism having them. Many diseases are recessives and it may take several generations for a recessive disease to appear. It is hard to6track recessives for this reason. For example, a family may be surprised that sickle-cell8anemia is in their genetic history. Family members Figure 6.6 Testcross for color on pea plants. A testcross always uses a homozygous recessive to attempt to reveal the recessive of its dominant mating partner. If even one of the offspring shows recessive characteristics, then the dominant partner harbors a recessive allele. In the example shown Y = yellow and y = green coloration in pea plants. ch06.indd 200 11/12/15 4:31 pm Chapter 6: Inheriting Genes may think it is not a risk because no one has had sickle-cell anemia, for as long as they can remember. However, it may have been hidden in the heterozygote condition for a period of time and was unexpressed. Deleterious recessives are a difficult but common thread in most groups. Some forms of porphyria are recessive, showing up many generations after they are thought to be gone. Did our narrator in the story experience the ­reappearance of a long-silent recessive allele? Meiosis: How Sex Cells Are Formed We have seen how traits are passed on from one generation to the next; now we will examine how organisms reproduce sexually. During Mendel’s time, it was accepted that parents transfer their hereditary information through a process called reproduction, to form a new organism. The central step in reproduction is fertilization, when a male and a female sex cell, both called gametes, unite. The female S sex cell is the egg; the male, the sperm. Each contributes half the total genetic material that unites and recombines in the zygote. If the offspring receives genetic materialM from both parents, how is it that I as the parents? The answer is the offspring contains the same number of chromosomes meiosis, which is a special form of cell division in which T the newly produced daughter cells contain only half the number of chromosomes of the parent. This half-quantity is H of genes in all of our other called the haploid or N condition, while the full complement cells, called somatic cells, is known as diploid or 2N. If,a sex cell were not haploid, then the genes in the sex cells would double with each successive generation. Every species has a set number of chromosomes. A mosquito has six chromosomes per cell; a sunflower, 34; a human, 46; a dog, 78; and aJlittle goldfish, an impressive 94 chromosomes. In contrast, gametes of each of these species O contain only half of these numbers: a mosquito gamete has 3 chromosomes, a sunflower, 17; a human, 23; a dog, S 39; and a ­goldfish, 47 chromosomes. H like a pair of shoes (see FigIn a diploid cell, each chromosome has a partner, much ure 6.7. The chromosome partners are known as a homologous pair. Homologs have one U maternal and one paternal copy of a chromosome. Alleles on each homologous chromoA some code for the same trait. An allele for eye color, for example, on one chromosome codes for eye color alongside the allele on its homologous pair. Figure 6.7 shows the 6 8 9 0 B U 201 Fertilization Is the process in which male and female sex cells unite. Gametes Reproductive cells (not given in bold in text). Zygote A fertilized egg cell. Meiosis A special form of cell division in which the newly produced daughter cells contain only half the number of chromosomes of the parent. Haploid (N) The half number of chromosomes of the parent. Somatic cells The full complement of genes in all of other cells. Diploid (2N) The full complement of chromosomes in all body cells (except sex cells). Homologous © 2006 by Kendall Hunt Publishing Company. Reprinted by permission The chromosome partners in a diploid cell. Figure 6.7 Homologous genes, knows as alleles, occur at the same location and code for the same traits. From Biological Perspectives, 3rd ed by BSCS. ch06.indd 201 11/12/15 4:31 pm 202 Unit 2: Is it all in the Genes? alleles on a chromosome (in varied colors). Before meiosis and mitosis take place, homologous chromosomes are duplicated. Thus, each replicated pair is composed of two sister chromosomes, identical to each other. Each set of duplicated homologous chromosomes contains four strands altogether: two original homologs and two duplicated strands. Each homolog of the pair contributes one allele for a trait to its offspring. As shown in Figure 6.8, homologous chromosomes separate into four gametes during production of sex cells. Whether the individual homologue gets into a sperm or egg depends upon S M I T H , 6 8 9 0 B U © 2006 by Kendall Hunt Publishing Company. Reprinted by permission J O S H U A Figure 6.8 Meiosis. Homologous chromosomes separate eventually into four sex cells (gametes). The doubling of genetic material takes place before the parent cell is able to divide. From Biological Perspectives, 3rd ed by BSCS. ch06.indd 202 11/12/15 4:31 pm 203 Chapter 6: Inheriting Genes chance, as described by Mendel. During meiosis, homologous chromosomes separate and move into one or the other of the gametes produced. They have an equal chance of entering a newly formed gamete because chance determines their entrance. Homologous chromosomes are inherited separately, as shown by Mendel’s law of segregation. Trace the movement of replicated chromosomes in Figure 6.8 to find the gametes’ destination. The diploid number (2N) of chromosomes in a parent cell is divided equally into the sex cells during meiosis. Thus, the halving effect on the chromosome number occurs during gamete formation. The result is a set of haploid (N) sex cells. This halving effect counteracts fertilization, which unites genetic material from two sex cells into one somatic cell, the zygote or fertilized egg cell. The result is a unification of N + N = 2N. As demonstrated in Figure 6.9, meiosis follows a series of stages similar to those seen in mitosis. Indeed, the names are also the same for the phases in both mitosis and meiosis. There are a few differences: 1) In meiosis, there are two sets of the same series of Sstages, meiosis I and meiosis II; but only one series in mitosis. This results in two cell divisions in meiosis and only M one cell division in mitosis. I as a result of the two divisions, 2) In meiosis, four new daughter cells are produced while only two are produced by mitosis. T 3) Each daughter cell contains only the haploid number of chromosomes in meioH sis, but daughters in mitosis contain the diploid. , in part because homologous 4) Gametes contain a variety of genetic possibilities, chromosomes separate into one or another of the sex cells, forming innumerable combinations. 6 8 9 0 B U (a1) © Alila Medical Media/Shutterstock.com J O S H U A Figure 6.9 a. Phases of meiosis. There are two stages of meiotic cell division, I and II. The end result of meiosis is the production of four haploid gametes (sex cells). Meiosis occurs in eight stages with descriptions of each stage given in the figure. b. Mitosis occurs in one division and results in two identical cells. ch06.indd 203 11/12/15 4:31 pm 204 Unit 2: Is it all in the Genes? S M I T H , 6 8 9 0 B U (a2) © Alila Medical Media/Shutterstock.com J O S H U A Figure 6.9 (continued) ch06.indd 204 11/12/15 4:31 pm Chapter 6: Inheriting Genes 205 Interphase Prophase Anaphase © Alila Medical Media/Shutterstock.com Metaphase S M Cytokinesis I T H (b) , Telophase Figure 6.9 Table 6.1 Meiosis I (continued) Comparison of meiosis and mitosis Number of cells Number of divisions Genetics of cells Compared to parents and each other Courtesy Peter Daempfle. J O Meiosis S Four new cells produced Two cell divisions H Haploid cells madeU Different (variability) A Mitosis Two new cells produced One cell division Diploid cells made Identical The Phases of Meiosis 6 8 In a period before meiosis, the interphase carries out functions similar to those during the interphase before mitosis: cells grow in size, organelles duplicate and grow, and 9 genetic material doubles in the nucleus. When genetic material doubles during inter0 phase, two pairs of homologous chromosomes are formed. The purpose of meiosis is B this, fertilization requires two to produce daughter cells capable of fertilization. To do haploid cells with haploid genetic material to unite. U During the first series of stages of meiosis, called meiosis I, homologous chromosomes separate (refer Figure 6.9 to see each stage). Just like the first stage of mitosis, when a cell begins meiosis, nuclear material condenses, transitioning from chromatin to chromosomes, its nuclear envelope disappears, and chromosomes attach to a spindle fiber. Unlike mitosis, the first stage of meiosis I, called prophase I, homologous chromosomes in proximity to each other exchange genetic material through a process called crossing over. In crossing over, segments of one chromosome swap with segments of another pair. Crossing over enhances the genetic combinations possible in gametes, as shown in Figures 6.8 and 6.10. Areas that are crossed over randomly swap genetic material, leaving each homolog with a unique set of DNA. In metaphase I, the homologous chromosomes line up as pairs, which later separate and move to opposite poles during anaphase I. Spindle fibers pull the pair of duplicated homologs into the center. In the next phase, anaphase I, homologous chromosomes ch06.indd 205 The process of cell division by which homologous chromosomes separate and new cells are haploid. Prophase I Also called the first stage of meiosis I, in which homologous chromosomes in proximity to each other exchange genetic material through a process called crossing over. Crossing over The exchange of genes between chromosomes. Metaphase I The stage of mitosis and meiosis that follows the prophase stage and precedes the anaphase stage (not given in bold in text). Anaphase I The stage of cell division in meiosis in which homologous chromosomes separate. 11/12/15 4:31 pm 206 Unit 2: Is it all in the Genes? separate to opposite ends of the cell. They are pulled apart in a random manner. A paternal homolog may be pulled onto one side while a maternal homolog may be pulled onto another side. At this point, the developing cells are haploid – with half the number of a complete set of chromosomes. With 23 sister chromosomes pairs, there are 2n possible new combinations. Thus, with 23 pairs of chromosomes in humans, there are 223 new possible genetic combinations in each newly formed gamete: 2 × 2 × 2 × 2 . . . 23 times! The genetic variation produced by random assortment is enormous. Mendel hypothesized this random segregation of chromosomes, long before an understanding of the phases of meiosis. Thus, three sources of genetic variation among organisms are seen: 1) meiotic segregation of chromosomes; 2) random mutations in genes as discussed in Chapter 5; and 3) crossing over, as discussed earlier in this section. The processes of obtaining genetic variation are shown in Figure 6.10. S M I T H , 6 8 9 0 B U (a) (b) © 2006 by Kendall Hunt Publishing Company. Reprinted by permission J O S H U A Figure 6.10 Genetic variation is introduced in species, especially during meiosis. a. Crossing over. b. Mutation. Recombination and independent assortment during segregation of alleles. All of these mechanisms add genetic diversity to cells and organisms. From Biological Perspectives, 3rd ed by BSCS. ch06.indd 206 11/12/15 4:31 pm Chapter 6: Inheriting Genes After this, telophase I and cytokinesis reform the nuclear envelop, with two new daughter cells containing their own nucleus. These new cells are haploid or N, containing only half the original number of chromosomes. When homologs are pulled apart during meiosis I, sister chromosomes are placed in daughter cells. The genetic composition of sister chromosomes is identical for the two. Thus, for these daughter cells, it is like getting two left shoes instead of a right and left. The two daughter cells of meiosis I are haploid, but contain a double set of half of the chromosomes. Separating of sister chromosomes occurs during the next series of stages of meiosis, called meiosis II. A short period separates meiosis I and II in a brief interphase. In this time, there is no new duplication of genetic material and quickly cell division resumes into prophase II. Chromatids reorganize, coiling tightly once again as chromosomes, in preparation for the pulling apart process. During metaphase II, chromosomes line up singly and then the two sister chromatids (which are identical In anaphase II, identical chromosomes separate, pulled apart by spindle fibers to opposite poles. In the last S phase, telophase I, nuclei reform and chromosomes become tightly coiled once again. The physical separation of cytoplasm takes place duringMcytokinesis, as it pinches off to become two new cells. The end result of telophase II and I cytokinesis, in which two new nuclei and cells form, is a total of four new haploid or N daughter cells. T As a result of meiosis, each human gamete contains only a haploid, 23 single strands of chromosomes, much like having 23 “left” shoes. It isHfertilization by another gamete, containing 23 “right” shoes, that gives new life with a full , diploid set of 23 pairs of chromosomes. Figure 6.11 shows chromosomes during meiosis represented as shoes. J Male and Female Gametes O S In animals, male meiosis produces four new sperm; and in females, one egg and three H of male and female meiosis. polar bodies form. All four gametes are haploid as products Their nuclear material is evenly divided in both sexes. U However, cytoplasm is unevenly divided in females. During a female’s telophase I, most of the cytoplasm is retained in A one daughter cell, leaving the other three with very little cytoplasm. Telophase I The stage resulting in the forming of a set of new cells. Cytokinesis The division of cell cytoplasm following mitosis or meiosis. Meiosis II The stage in which sister chromosomes are separated. Metaphase II The stage in which chromosomes line up singly and then the two sister chromatids separate and move to opposite poles of the cell. Prophase II The first stage of meiosis II. Telophase II The last stage in the second meiotic division of meiosis. Telomere Chromosome Cell Nucleus © Kendall Hunt Publishing Company 6 8 9 0 B U 207 Figure 6.11 Chromosome separations of “shoes.” ch06.indd 207 11/12/15 4:31 pm 208 Unit 2: Is it all in the Genes? • Although note that not all animals reproduce sexually. For example, in ants, bees and wasps, a virgin birth (parthenogenesis) takes place to produce males, which will be discussed in Chapter 20. During the next meiotic division, another unequal partition of cytoplasm happens. In most female animals, the result is a set of three small sex cells and one large sex cell. The daughter obtaining most of the cytoplasm becomes the female egg, while the others become polar bodies. Polar bodies generally disintegrate quickly and are not viable for fertilization. In human males, four gametes are made per meiotic division (Figure 6.12). However, many divisions occur simultaneously, continuously producing large numbers of gametes. The average ejaculation contains about 225 million sperm. In females, there is generally only one egg in a cycle. A great deal of energy is placed into egg production, but sperm are made en masse. In most plant and animal species, the female gamete contains most of the cytoplasm. Can you deduce why? The egg will S provide most of the resources, both nutrients and organelles, for a developing zygote. M Once fertilized, an egg has the full complement of genetic material from unification with a sperm. Its cytoplasm provides an excellent I T Oogenesis H Oogonium , Primordial follicle 2n J O Before birth S Childhood - ovary inactive H puberty to menopause From U Primary A egg Primary egg arrested in prophase I 2n 2n Meiosis I n First polar body (dies) n n Meiosis II (completed only if fertilized) n Second polar body (dies) 2n 6 8 Secondary egg 9 0 BSecondary egg, arrested Uin metaphase II, ovulated Primordial follicle Primary follicle Growing follicle Mature follicle Ovulation Corpus luteum Zygote © Alila Medical Media/Shutterstockcom Mitosis Follicle development (a) Figure 6.12 Gamete development. a. Females who carry out oogenesis (egg ­formation) and b. males, who carry out spermatogenesis (sperm formation). Both result in the production of four haploid gametes, but males produce four sperm (in the tubules of the testes) and females produce one viable egg with three polar bodies (within ovaries). ch06.indd 208 11/12/15 4:31 pm Chapter 6: Inheriting Genes 209 Spermatogenesis Seminiferous tubule Type A spermatogonia 2n Mitosis Type A spermatogonium 2n 2n Type B spermatogonium 2n Primary spermatocyte n Secondary spermatocytes n Meiosis II n n n n Spermatids (2 stages of differentiation) n S M I T H , Spermiogenesis n J O S (b) H (continued) U A Spermatozoa Figure 6.12 Lumen © Alila Medical Media/Shutterstock.com Meiosis I nutrient resource for the new organism’s survival. Also, by having instant organelles on hand for growth and development, the new embryo has an advantage. This is why 6 inherit, in accordance with the mitochondria and chloroplasts are so beneficial to simply endosymbiotic theory – instant energy and food production 8 for a new organism. 9 0 Why sex? It has its advantages and its disadvantages for a species. Asexual reproduction is more efficient and requires less cell machinery. B Prokaryotes reproduce by binary U fission, simply splitting in half to form two new organisms. The main disadvantage Sex: A Cost–Benefit Analysis of asexual reproduction is limited genetic variation. Asexual reproduction perpetuates the genotypes of its parents, changing very little from generation to generation. Sexual reproduction instead, leads to many varieties of offspring, enabling some organisms to survive during changing conditions. If, for example, a change in the environment should occur, as in the potato famine in Ireland in the 1800s, all asexually produced offspring will respond in the same manner. In Ireland, all of the potato plants at the time were grown asexually from the same original plant. The organisms produced, with the same genetic variety, were susceptible to the same fungal-like protist, causing them to decay and leading to famine. Genetic variation allows for differences in a group so that at least some will survive. Variety in potatoes in Ireland at the time would have saved over two million lives. ch06.indd 209 11/12/15 4:31 pm 210 Unit 2: Is it all in the Genes? Sexual reproduction, on the other hand, allows new combinations of genes to form in offspring. Through crossing over, segregation, and mutation, many genetic combinations are possible. Of course, asexual reproduction allows for some variation due to random mutations in organisms’ gene sequences during replication, but overall it results in limited variety. Sexual reproduction gives a survival advantage in the process of ­evolution – it provides enough genetic variation among individuals to help them adapt, as a species, to environmental changes better than asexually reproducing organisms. The Benefits of Sex is Debatable When observing the praying mantis’s mating ritual, in which a male has innate fear of its mate, one still wonders if it is all worthwhile. One hypothesis conS tends that a female bites his head off during copulation (the act of sex), in order to “ease his mind” andMrelax during sex (Figure 6.13). This allows more of his sperm to enter into her. I She is not wasteful, and eats his whole body after sex in order to gain energy for her developing embryos. Sex can be very efficient in its quest to build T a better species. H , Determining Sex Sex chromosome The final smallest pair of the 23 pairs of chromosomes in humans. Y chromosome A sex chromosome that is found only in males (not given in bold in text). J Upon closer inspection of the 23 pairs of chromosomes in humans, the final smallest O pair are the sex chromosomes. The other 22 pairs are called autosomal chromosomes, S Human sex chromosomes are either X or Y chrowhich carry out a cells’ life functions. mosomes, and these determine the H sex of an organism. If a human has both an X and Y chromosome, or is XY, it is male; and if it has two X chromosomes, or is XX, it is U from each other in a number of ways: a Y chromofemale. The sex chromosomes differ some is much smaller than an X; aAY chromosome carries very little genetic information; and a person can survive without a Y chromosome. After all, human females carry only two X chromosomes. A karyotype, which shows a visual map of a set of chromosomes for an organism, is given in Figure66.14. In some disorders, chromosomes fail to separate © 2happy/Shutterstock.com 8 9 0 B U Figure 6.13 Praying mantis sex. Soon after copulation, she will bite off his head and consume his body for the energy to raise their young. ch06.indd 210 11/12/15 4:31 pm Chapter 6: Inheriting Genes 211 Human karyotype 2 3 6 7 8 13 14 15 19 20 Figure 6.14 21 4 9 10 16 17 22 5 11 12 18 Y X © Zuzanea/Shutterstock.com 1 S Human karyotype. M I example, in disorders such as and an abnormal number of chromosomes are seen. For Down syndrome, an extra chromosome #21 is found T after the failure of that chromosome to separate during meiosis. H In ants, bees, and wasps, in the order hymenoptera, queens produce haploid males , and diploid females, making females more related genetically to each other than to 6 8 9 0 B U © Dariusz Majgier/Shutterstock.com males. In fact, they share 75% similarity in DNA, because females have all of the same genes in common from their fathers. The father is haploid and has only one set J of the same chromosomes to give to all of his daughters. This phenomenon is known as haplodiploidy, in which some offspring are haploidO and some are diploid. This is a basis for close relationships in ants, bees, and wasp societies: they share duties very S closely within colonies. In fact, most females within a colony give up sex altogether, H remaining sterile castes whose main purpose is to serve the queen master (Figure 6.15). Many plants and all earthworms have both male and U female parts; they produce both male and female gametes. Sometimes simple temperature A determines sex, as in turtles, lizards, and reptiles. In turtles, cooler temperature eggs become males, while warmer ­temperatures elicit females. Figure 6.15 Worker ants helping their queen. Loyalty is strong for a queen who controls all aspects of ant society. In this image, worker ants move their queen’s eggs, serving both their queen and their future sisters who will hatch from those eggs. ch06.indd 211 11/12/15 4:31 pm 212 Unit 2: Is it all in the Genes? Sex Is Not Sexual Preference Neurotransmitter Are chemicals that affect different parts of the brain. Hypersexuality The condition in which one has many sex partners. Asexuality The lack of sex drive. Kin selection The theory that evolution favors helping between family members or kin to augment the transmission of their related genes. Most research supports a strong genetic basis of sexuality. Behavioral genetics is the research specialty that studies the genetic basis of behavior, including sexual preferences. Sexual drive and desire vary across a continuum in most animal societies from asexual (no sex) to hypersexual (excessive sex). It is not a simple like or dislike of certain attributes in the opposite sex. Studies of monozygotic (identical) twins show high contributions of genetic influences for sexual preferences. Biological bases for sexuality lie in two factors in animals: 1) activity of the medial preoptic area of the brain (MPOA) and 2) DRD4 dopamine receptor gene. Dopamine is a neurotransmitter found in the brain. Neurotransmitters are chemicals that affect different parts of the brain. In humans and rats, for S example, the greater the activity in the MPOA area and the greater the number M of DRD4 receptors, the higher the sexuality rates in humans and rats. I and behaviors makes sense evolutionarily. A range in sexual drives ­ ypersexuality, or having many H T sex partners, may appear favorable for enhancing one’s reproductive success (more offspring with more partners), but this is not H so—quality also counts. Consider that after fertilization, in many animals a seminal , If another partner enters, the plug is dislodged plug forms after a male ejaculates. and this next partner is also able to produce a viable offspring. In promiscuity, the final partner is equally likely to father the child as compared with the first partner. J Usually the last partner in is weaker, older, and has poorer quality genes than the O first. In animal systems, hypersexuality is therefore selected against, with many partners leading to weaker offspring. Experimental ­ evidence shows that hyperS sexual behavior in rats leads to decreased reproductive ­success for the female. H At the other extreme, asexuality, which is a lack of sex drive, is observed in U such genes persist? One obvious answer is that about 1% of humans. Why do a lack of sexual attraction does A not mean lack of sexual behavior. On the other side, one would also expect homosexuality to be selected against as it does not lead to new offspring. Another hypothesis as to why 6 pool is based on kin selection. Kin selection “gay” genes remain in our gene is the theory that evolution 8favors helping between family members or kin to augment the transmission of their related genes. People who do not have their own children are more likely9to help their nephews and nieces (kin), who are 25% identical to them. This 0 behavior perpetuates their own genes more than not having any children. Thus, B there is strong evidence for a genetic basis of sexual preference and helping behaviors. U Mendelian Traits: Single Gene Characteristics Mendelian ­characteristic ­(single-gene trait) Traits that are determined by instructions on a single gene. ch06.indd 212 Mendel did not yet know about molecular structures and the chemical idea of the gene discussed in chapter 5, but his explanation for their transmission was remarkably accurate for many traits: that there is pattern to their heredity and that they are inherited as discrete units. Traits that are determined by instructions on a single gene are called Mendelian characteristics, or single-gene traits. There are more than 9,000 single-gene human traits that follow the principles of Mendelian genetics. These are either-or characteristics: an organism has either one type or the other. 11/12/15 4:31 pm 213 (b) © Piotr Marcinski/Shutterstock.com © Ozgur Coskun/Shutterstock.com (a) © Everett Collection/Shutterstock.com Chapter 6: Inheriting Genes (c) Figure 6.16 Examples of single-gene traits. A variety of characteristics are controlled by a single gene pair. Tongue rolling, for example, is dominant over not being able to roll one’s tongue and attached ear lobes are recessive. What Mendelian characteristics do you have? a. Colin Farrel, shown here with his sister, has a widow’s peak. b. This father and son both exhibit the tongue S rolling ability. c. This man’s ear lobes are attached. 0 B U Sex-linked One of the three possible patterns of inheritance of singlegene traits in which the X chromosome determines the characteristic. X chromosome A sex chromosome that is found twice in females and singly in males (not given in bold in text). Autosomal dominant The patterns of inheritance of singlegene traits in which the dominant allele gets expressed. © 2006 by Kendall Hunt Publishing Company. Reprinted by permission M Consider being able to roll your tongue or not roll your tongue; having a widow’s I peak or not having a widow’s peak; and having albinism or not having albinism. Each T sets of alleles. A person who is determined by whether one has dominant or recessive has a widow’s peak has a dominant allele dictating thatHthe characteristic will show up. Figure 6.16 illustrates a few single-gene traits. , There are three possible patterns of inheritance of single-gene traits leading to an organism’s outward appearance: 1) autosomal dominant, in which the dominant allele gets expressed, 2) autosomal recessive, in which bothJrecessive alleles are present for a person to get the recessive trait, and 3) sex-linked, in which the X chromosome deterO Mendel’s rules, expressing mines the characteristic (Figure 6.17). Each pattern follows S for each pattern are given in the dominant allele in the phenotype. Examples of traits Figure 6.17. H Autosomal Dominant. Diseases that are autosomal dominants are expressed when U in Huntington’s disease, a even one allele is contained within a genotype. For example, degenerative and progressive muscular illness, the trait A is inherited as a dominant allele. If a person receives the autosomal dominant Huntington gene, she or he will develop its related disease. Symptoms usually develop after an age of 30 years, well after she or he 6 could pass it onto children. Singer Woody Guthrie, who sang “This land is Your 8 Land,” died from the disease at an age of 55 years, 13 years after symptoms appeared. 9 He was the father of singer Figure 6.17 Examples of traits in three patterns of inheritance: autosomal dominant, autosomal recessive, and sex-linked traits. Each method of inheritance depends upon the expression of genes. Pedigrees for each pattern of inheritance give affected and normal individuals in each generation. From Biological Perspectives, 3rd ed by BSCS. ch06.indd 213 11/12/15 4:31 pm 214 Unit 2: Is it all in the Genes? Autosomal recessive The patterns of inheritance of singlegene traits in which both recessive alleles are present for a person to get the recessive trait. Arlo Guthrie, who did not inherit the disease from his father. Arlo had a 50:50 chance of getting Huntington’s disease. Its origin is thought to have arisen from a small town in Venezuela. About 30,000 Americans suffer from the disorder today. Autosomal Recessive. Most diseases are carried on recessive alleles. Recessive alleles stay hidden within a genotype without being expressed for longer periods of time than autosomal dominants. As discussed earlier in this chapter, a person may harbor a recessive allele without knowing it is present; the dominant allele covers its effects within the genotype. Thus, deleterious recessives persist in groups. For an autosomal recessive trait to be expressed, an individual must inherit one recessive allele from each parent. Thus, two unaffected individuals have a 25% chance of having an affected child. In Xeroderma pigmentosum, lack of DNA repair enzymes due to recessive alleles leads to skin lesions and skin cancers While certain forms of porphyria as described in our story are inherited in an autosomal dominant pattern (AIP), other forms occur through an autosomal recessive pattern S (congenital porphyria). In both forms, those affected lack enzymes to produce heme M oxygen is carried throughout the body by heme groups in red blood cells. Because groups lack of heme causes damage I to body systems. (Human systems will be discussed in later chapters.) Both dominant and recessive porphyria are difficult to treat because T insufficient blood causes irreversible damage to vital organs. In January 2013, it was H King George III of England were discovered. His reported that the remains of the mad mental health as a leader was in ,question throughout his reign. King George III likely suffered from porphyria. His mental deterioration and decline are chronicled in the 1994 film, The Madness of King George. Many of the royal families married kin; increasing J such as porphyria. chances for inheriting harmful genes, Sex-Linked. Sometimes malesO have a greater chance of inheriting a trait than females. This occurs in sex-linked traits, in which a trait is determined by a gene located on a S patterns different between males and females. In sex chromosome, making inheritance H sex-linked traits, such as in color-blindness, often the disease-causing allele is recessive. Most genes are found only on theUX chromosome, so it determines the expression of a trait. A blindness, for example, she will not become color If a female has one gene for color blind if she has another dominant, normal gene on her other X chromosome. The dominant allele masks the recessive 6 allele causing color blindness. Alternatively, the same situation in a male would result in color blindness. A male does not have two X chro8 recessive gene. Because a male has a Y chromomosomes to hide the one troublesome, some, which has very little genetic 9 information, it does not hide the effects of the normal dominant allele. Sex-linked traits0are more common in males than in females because of this pattern (see Figure 6.18). Females have greater opportunity to hide alleles with B genes from their other X chromosome. U Not So Mendelian Genetics Most traits do not act as Mendel predicted. How do we explain why there is not simply one or two possible skin colors? If all traits were Mendelian, all organisms of a species would have either one phenotype or another, with no variations in between. Obviously, this is not the case for most organisms’ characteristics. Other inheritance patterns produce the phenotypes most common to us: skin color, IQ, blood types, height, weight, and sexual preference to name a few. While Mendel had great insights into his data, most of our genetic expression is more complex than the seven pea plant traits he chose to study. ch06.indd 214 11/12/15 4:31 pm (a) S M I T H the, X (b) 215 © 2006 by Kendall Hunt Publishing Company. Reprinted by permission Chapter 6: Inheriting Genes Figure 6.18 a. Sex-linked traits. Inherited on chromosomes, they are more likely to appear ­phenotypically in males. b. Snapdragons. Red and White Cross of F1 generation results in pink plants. 50% of the offspring exhibit incomplete dominance, showing a pink coloration. This phenotype was not seen in its J parents. From Biological Perspectives, 3rd ed by BSCS. O S H Incomplete Dominance U contributing to gene expresIncomplete dominance results from two different alleles sion. Snapdragon plants, for example, occur in red and A white varieties, but may produce pink flowers when mated together. A cross between a white and red Snapdragon plant is shown in the Punnett square in Figure 6.18. The red and white alleles are equally expressed in snapdragons, resulting in a pink color. 6 8 Multiple Alleles 9 Some traits are controlled by several genes, each expressing a particular phenotype. 0 These traits are examples of multiple allelism. Individuals still carry only two of the B multiple alleles at any one time, one from a father and one from a mother. However, U blood groups, there are three the traits are all expressed within a population. In human alleles controlling blood types: allele A, allele B, and allele O. Alleles A and B are codominant, or share dominance with each other, and allele O is recessive. When allele A or B are present with O, as in AO or BO, the result is a blood type of A or B, respectively. When A and B are inherited together, a blood type AB results, and when allele O is homozygous with OO as the genotype, the result is blood type O. Alleles code for antigens, or special proteins on plasma membranes of red blood cells: allele A codes for an A antigen, allele B codes for a B antigen, and allele O codes for no antigen. Antibodies are chemicals made by the immune system that initiate an attack on foreign bodies. When blood types with foreign antigens mix, antibodies are made against antigens found on red blood cells. ch06.indd 215 Incomplete dominance A genetic situation in which one allele does not completely dominate another allele. Multiple alleles A series of three or more alternative forms of a gene, out of which only two can exist in a normal, diploid individual. 11/12/15 4:32 pm 216 Unit 2: Is it all in the Genes? Universal donor A person of blood type O who may donate blood to any other blood group because the blood group contains no antigens on its red blood cells. Universal recipient A person of blood type AB who may receive blood from any other blood group because the blood group contains all antigens on its red blood cells. To apply this, blood type O may be donated to any other blood group because it contains no antigens on its red blood cells for which to attack. Blood type O-is therefore called the universal donor. Blood type AB contains both A and B antigens on the red blood cells. Therefore, a person with blood type AB is able to receive all other blood types because they appear non-foreign to an AB immune system – all of the antigens are already on its red blood cells. Blood type AB+ is therefore called the universal recipient. Blood type A cannot donate to blood type B and vice versa. Blood type A has A antigens and makes antibodies for B (because B appears foreign to it). Blood type B has B antigens and makes antibodies for A (because A appears foreign to it). Note that “+” and “−” have been used to describe blood types. Blood is classified as either positive “+” or negative “−” because of a surface protein marker on red blood cells, called the Rh factor. If blood contains an allele coding for the Rh marker, then its blood is considered positive. Type A+ blood contains at least one allele for the A antigen and one allele for the Rh factor. The Rh marker is another substance for immune cells to S recognize and attack. Those with Rh positive blood types are able to receive Rh negative M however, are not able to receive Rh positive blood. blood. Those with Rh negative blood, Rh positive blood contains the Rh I marker, which would be recognized and rejected by immune cells of an Rh negative person. Figure 6.19 shows the four blood types along T with their antigens and the red blood cells associated with each blood type. H Polygenic Inheritance , Antigen A Antigen B Anti-B Antibody Anti-A Antibody Type A Type B 6 8 Antigens A and B 9 0 B U Red Blood Cells Neither Antigen A nor B Neither Anti-A nor Anti-B Antibodies Anti-A and Anti-B Antibodies Type AB Type O Plasma (a) (b) © 2006 by Kendall Hunt Publishing Company. Reprinted by permission Are traits with patterns of inheritance determined by more than one gene and influenced by the environment. Most of an organism’s characteristics are polygenic traits, which are traits with patterns of inheritance determined by more J than one gene and influenced by the environment. These include height, skin color, eye color, weight, hypertension, cancer, and heart disOcontinuous, with many levels expressed along a bellease. Polygenic traits are said to be shaped curve. Figure 6.20 shows S the curve for height in athletes as they have changed in the past century. Both exhibit a polygenic bell shape, but the average has increased conH siderably. What factors in society have changed to increase average height in our society? U Dominance or recessive expression is not so clear cut for polygenic traits. We are A a smart or a bad student or even a brown or blue not either short or tall, strong or weak, eye color. There are many variations in between these extremes. Most individuals cluster around an average with very few found at the extremes. © Kendall Hunt Publishing Company Polygenic traits Figure 6.19 Codominance and multiple alleles. a. There are four discrete blood types in humans: A, B, AB, and O. Three different alleles determine blood type. Blood type is expressed as codominance with alleles sharing a phenotypic expression. b. Genetics of the human ABO blood groups. From Biological Perspectives, 3rd ed by BSCS. ch06.indd 216 11/12/15 4:32 pm 217 © Courtesy of The Library of Congress. Chapter 6: Inheriting Genes Figure 6.20 People are often categorized by their height. The mean height of men today is 5'10", whereas in 1913 it was 5'8". The photo from 1913 shows a group of college students categorized by height. Note that the categories follow a bell-shaped curve, a characteristic of polygenic traits. What factorsSdo you think contributed to the change in average height over the past century? M I Polygenic traits are influenced by the environment because genes alone do not explain T traits because they have many the variation in phenotypes. They are called multifactorial factors that affect their expression. Environment interactsHwith genes to form a phenotype. Obesity, a polygenic trait, was studied to determine the effects , of genes and the environment on its expression in humans and mice. Identical twins, which have the exact same genotypes because they arise from the same fertilized egg, were studied. Twin studies often measure how much a polygenic trait is due to genetics. Obesity J had a concordance rate of 70%, meaning that 70% of the time obesity is found in both twins, O regardless of what they ate. The mouse Ob gene encodes for a weight-controlling hormone, leptin, produced in S a mutated Ob gene, and one fat cells. Figure 6.21 shows two mice, one overweight, with normal weight, with a normal Ob gene. The human gene H for leptin is on chromosome #7 and its mutation increases the risk for developing obesity. However, a mutation of U the leptin gene is not the only contributor to obesity. Obesity is a complex disorder, A involving the interaction of several genes with the environment. Indeed, scientists have detected genes for obesity in humans on chromosomes: #2, #3, #5, #6, #7, #10, #11, #17, and #20. Research on this multifactorial condition continues. (b) © ARTSILENSE/Shutterstock.com (a) © Vasiliy Koval/Shutterstock.com 6 8 9 0 B U Figure 6.21 (a) Normal vs. (b) chubby rat, the ob gene has its effects on weight in rats (normal rat on the left and obese rat on the right.) ch06.indd 217 11/12/15 4:32 pm 218 Unit 2: Is it all in the Genes? In our story, porphyria symptoms emerge from genetic and environmental factors. While there is a genetic component, stress, smoking, alcohol, and sun exposure trigger symptoms of porphyria. It is also shown that garlic aggravates porphyria symptoms, possibly the root of the assertion that garlic keeps vampires away. There are eight enzymes involved in heme biosynthesis. Each enzyme has genes that code for it. If any one of these genes is mutated, abnormal heme production results. Thus, the disease has genetic roots as well as environmental triggers. It is multifactorial because many (or multiple) factors affect its expression. Some polygenic traits are due to gene–gene interactions with very little environmental input. For example, eye color is influenced by about 16 different genes, with less than 1% of its phenotype due to the environment (Figure 6.22). You may have assumed that eye color is an either/or scenario, but in fact it is a polygenic trait, with a continuum of colors possible. Have you ever wondered how hazel or green eyes develop? It is a matter of pigments. The more genes inherited for pigmentation in the eye’s iris, the darker the S coloration. If there are no alleles for pigment production in one’s genotype, eyes will be blue; if there is one or two genes, M eye color will be green; if there are three or four alleles for pigment, coloration will be hazel, I and more alleles for pigment give varying shades of brown. T H Pleiotropy , When one gene affects more than one trait, this effect is called pleiotropy. Several spe- Pleiotropy The condition in which one gene affects more than one trait. cies of farm birds – chickens, turkeys. – exhibit a “frizzle” mutation on one of their genes. The frizzle allele causes J bird feathers to be stringy and weak, providing poor insulation. More seriously, the mutated frizzle allele affects the bird’s heart, kidneys, and O Pleiotropy is seen in many characteristics from thyroid and impairs its overall health. S with effects on brain and skin functions, to multiple phenlyketonuria (PKU) in humans, congenital deformities in rats. AllHof the associated features of the disorders are due to a single-gene effect on multiple traits. U Tracing Gene Flow inA Families: Pedigree Analysis Pedigrees are diagrams of genetic relationships among family members through different generations; they are used6to trace gene flow through a family (see Figure 6.23 as an example). They show patterns 8 and help figure out whether one has a dominant or © wavebreakmedia/Shutterstock.com 9 0 B U Figure 6.22 Eye color genotypes and phenotypes. Eye color is mostly written in our genes. ch06.indd 218 11/12/15 4:32 pm (a) 219 © 2006 by Kendall Hunt Publishing Company. Reprinted by permission. Chapter 6: Inheriting Genes (b) S tree. a. Symbols are used to create the family tree. Figure 6.23 A pedigree shows the genetics of a family M royal families of Europe. Hemophilia is a sex-linked This pedigree shows the inheritance of hemophilia by the trait. The Bettmann Archive. b. A pedigree of a familyI with congenital porphyria, a recessively linked trait. From Biological Perspectives, 3rd ed by BSCS T H recessive allele, based on one’s parents. The pedigree diagram uses circles to indicate females and squares for males. No shading indicates, unaffected individuals, shaded are affected and half shaded are known carriers or heterozygotes. Horizontal lines between circles and squares show mating and vertical lines show descent. Several J traits shown in Figure 6.23 indicate how genes are expressed through generations: pedigrees for ­hemophilia and a family with porphyriaO are given. S Tracing Gene Flow in Groups:H U Population Genetics A How do genes move between villages, cities, and continents? If you compare groups that are separated geographically do you find different characteristics? Is there such a concept as “race,” genetically separating different groups of 6 humans? These questions have answers in the branch of genetics called population genetics. A population is defined as a group of individuals able to breed with each other in a8given area, producing fertile off9 to another and within groups spring. The study of patterns of gene flow from one group is known as population genetics. 0 Among other things, population geneticists investigate how diseases are carried in a B population of organisms. Mathematical calculations determine the frequency of alleles U flow over time. Porphyria was in a group. These numbers help determine trends in gene found to be in high proportions in populations in the old Austro-Hungarian Empire’s province of Transylvania, where the myth of vampirism originated in our opening story. Further studies are being done to determine the exact origins. In the example of Huntington’s disease, however, population geneticists determined that the gene arose from one woman in a small town in Venezuela, according to records dating back to the 1700s. Scientists collected information from 90,000 people and developed pedigrees to chart gene flow. They tested blood samples to detect the disease and plotted its movement through the years. Though Huntington’s disease is inherited, a 2001 study indicated that roughly 10% of cases result from new, random mutations. Understanding how genes move within a population can help explain why certain genes persist in that population, and this in turn enables us to better understand diseases ch06.indd 219 Pedigree Are diagrams of genetic relationships among family members through different generations; they are used to trace gene flow through a family (not given in bold in text). Population genetics The study of patterns of gene flow from one group to another and within groups. 11/12/15 4:32 pm 220 Unit 2: Is it all in the Genes? and organism characteristics. For example, by mapping out where cystic fibrosis is located geographically, scientists determined its benefits to immunity against cholera. It is difficult to determine the exact number of carriers in a population because carriers exhibit a normal phenotype. However, scientists may use a mathematical formula to estimate the probability of occurrence of a recessive allele in a population. The Hardy– Weinberg quadratic equation for equilibrium shows the relative proportion of alleles in a population through counting the number of recessive individuals: p2 + 2pq + q2 = 1 In the equation, p equals the proportion of dominant alleles in a population and q equals the proportion of recessive alleles within a population. Homozygous dominant organisms are given as p2 and homozygous recessive are given as q2. Heterozygotes or carriers are given as pq. Through counting the number of dominant individuals, which one is S p is calculated. Then, q is solved for, and the rest able to detect through observation, of the equation’s letters are calculated M using the quadratic equation. This is a quadratic equation set equal to one. It assumes I that a population is not evolving or changing in its allele frequency. It assumes no immigration, emigration, natural selection, or mutations that alter normal gene frequency.T Obtaining data through use H of the Hardy–Weinberg equation helps determine the risk of having a particular gene within one’s population, helps understand if a popula, tion, such as a stand of red maple trees, is undergoing a change in gene flow, and examines how populations compare with each other based on genetic factors. For example, with respect to the alleles for sickle-cell anemia: African American populations with J West African ancestry have a 12.5% prevalence of sickle-cell anemia, but West AfriO can populations have a 20–40% prevalence. This indicates that the populations have S diverged in their overall genetic compositions. Genealogy is the study of family H history. It is related to population genetics, using pedigrees to investigate one’s family history. New tests are available that allow one U to send in a blood or saliva sample and have it analyzed to trace genetic origins. For A instance, tests identify over 400 different ethnics groups in Africa from which our genes may be compared to determine origins. Is this useful or does this further divide people based on the social construct of race? 6 8 9 0 B For Comfort or a Good Inbreeding:Too Close Strategy? U Consanguinity, or sharing blood through mating with close relatives, such as brothers and sisters, has been shunned by most societies throughout history. The cultural taboo has a practical origin: inbreeding depression, or the loss of heterozygotes and at the same time, the acceleration in the number of recessive alleles in a population that are often harmful. The Hardy–Weinberg equation shows the increase of both recessives and their related diseases in studies of inbreeding groups. Individuals in the same family share many genes in common. The recessive genes that would otherwise be covered up by the dominant allele are more ch06.indd 220 11/12/15 4:32 pm Chapter 6: Inheriting Genes 221 likely to become expressed when recessives occur more frequently. It is likely that pockets of porphyria existed in Medieval Europe, where intermarriage was somewhat common. Porphyria would have been more pronounced in such areas, where dominant normal alleles for heme formation were less prevalent. When close relatives mate, both are part of a lineage that has the potential of sharing more of the same harmful genes in common. Examples may include sickle-cell anemia, cystic fibrosis, or even cancer. On the other hand, recent research shows that a certain amount of inbreeding can produce healthier children. In a study of Iceland’s family history lineage, marriage between third and fourth cousins produced the most numerous and healthiest children over the past 1,000 years. It is hypothesized that outbreeding, or mating with someone too different from one’s own genotype, may also lead to health problems in children. In fact, about 20% of marriages S worldwide occur between first cousins. This practice is illegal in many of the M United States. Outbreeding too far also has negative consequences, though. One such I example occurs for the Rh factor, cited earlier in the chapter. Rh is a set of T protein markers on red blood cells that need to match between mother and child for a healthy baby to be born. If the mother isHRh negative, and the father is Rh positive, then the blood of the second fetus , who is Rh positive (from the father) will be recognized as an invader by the maternal immune system. Presently, Rhogam is a treatment given to pregnant mothers to prevent misJ Without modern techmatched blood from causing a problem (Figure 6.24). nology, however, such a match would be disfavored. O Thus, there is an optimal level of inbreeding for reproductive success. However, third and fourth cousins S have only about 1/256 to 1/512 genes in common with one another, so the chances of revealing recessive alleles is quite low. H U A © Lisa S./Shutterstock.com 6 8 9 0 B U Figure 6.24 blood types. ch06.indd 221 Rhogam is used to treat Rh incompatibility between mother and fetal 11/12/15 4:32 pm Unit 2: Is it all in the Genes? Biotechnology The branch of science that uses biological knowledge and procedures to produce goods and services for human use and financial profit. Gene technology The technology that modifies plants, bacteria and animals to create products for society. Genetically ­modified organism Are organisms in which DNA is genetically altered via genetic engineering techniques. Genetic engineering The process in which an organism’s genes are manipulated in a way other than is natural. Recombinant DNA technology The process by which DNA is extracted from nuclei of organisms and treated with restriction enzymes. Gene Technology: Solving Problems Using Genetics Biotechnology is the branch of science that uses biological knowledge and processes to produce goods and services for human use and financial profit. Its techniques manipulate genetic sequences in organisms to produce medical drugs and develop weatherand pest-resistant crops, to name a few examples. One significant sub-branch is gene technology, which modifies plants, bacteria, and animals to create products for society. First, the genome of a specific organism is modified by inserting a gene from another organism into the subject organism’s already existing DNA. The resulting organism is called a genetically modified organism (GMO) and it is classified as transgenic because it contains genes from another species. Inserted genes produce proteins, for which the inserted gene codes. Human proteins such as insulin, to help diabetics, human growth hormone or HGH, to help in dwarfism, and factor VIII to help hemophiliacs are proS duced by these GMO organisms. Transgenic tobacco plants produce HGH, as shown in M Figure 6.25. Before gene technology, the Iavailable means of collecting these proteins had many drawbacks. HGH was collected from T dead bodies and could cause disease when injected into patients, for example. Hemophiliacs, who suffer from life-threatening blood loss H due to the lack of a blood clotting factor, were dependent on blood transfusions, which carry a risk of containing infected,blood. Before AIDS was discovered in the early 1980s, many hemophiliacs were infected with HIV from blood transfusions. Gene technology changed their treatment options, leading to less risk. Hemophilia is now treated with J genetically produced clotting factor VIII. Lessened risk from disease-causing agents is O to gene technology. a great step forward for society due GMOs are produced through S genetic engineering, which is the manipulation of an organism’s genes in a way other than is natural. This manipulation is accomH plished through using a technique called recombinant DNA technology (Figure 6.26). the process by which DNA is extracted from nuclei of ­Recombinant DNA technology isU organisms and treated with restriction A enzymes. Restriction enzymes cut DNA at specific sequences. A bacterial plasmid, which is a circular strand of DNA, is also cut with 6 8 9 0 B U © Vasiliy Koval/Shutterstock.com 222 Figure 6.25 Transgenic tobacco plants. These plants are being used to produce human growth hormone (HGH) to treat human growth disorders. A gene has been inserted into these plants to produce HGH. ch06.indd 222 11/12/15 4:32 pm Chapter 6: Inheriting Genes 223 J O S H U A 6 8 9 0 B U © 2006 by Kendall Hunt Publishing Company. Reprinted by permission S M I T H , Figure 6.26 Genetic recombination techniques. They are steps used in producing a genetically modified, transgenic organism. Note that the restriction enzymes cut DNA at specific locations, allowing plasmid DNA to attach and become a “part” of the DNA of the newly created transgenic organism. In this figure, the clotting factor VIII gene is inserted into bacteria in order to produce factor VIII en masse for human use. The bacteria made by genetic recombination are genetically engineered “transgenic” o ­ rganisms. From Biological Perspectives, 3rd ed by BSCS. ch06.indd 223 11/12/15 4:32 pm Unit 2: Is it all in the Genes? © Nikolay Litov/Shutterstock.com 224 S 6.27 Figure Panhemin Vial. M the same restriction enzyme. Bacterial and human DNA fragments are mixed together, I causing them to link with each other. The bacterial plasmid now contains the human T new proteins. The plasmid is then transferred into gene that will be used for coding H cell expresses the newly inserted gene to make the a new bacterial cell. This bacterial desired protein. It divides over and , over, forming new cells that make the a p­ roduct. The Gene therapy The process in which genes are inserted into an organism to treat its disease. bacterium with its newly inserted gene is said to have been recombined. In our story, Herbie might benefit if biotechnology treatment options were available for porphyria. To date, porphyriaJ is treated with limited success, with symptoms and long-term problems plaguing its sufferers. An area of study that holds promise for more O successful treatment of porphyria and other diseases is gene therapy. Gene therapy is S to treat its disease. In the past two decades, gene the insertion of genes into an organism therapy has had mixed success. Future H research may find a way to insert a gene into porphyria patients such as Herbie, that U blocks the mutated gene, which is unable to produce normal heme groups. Another advance for porphyria sufferers would be in the area of A transfusions restore deficient heme in the blood of blood production. Presently, blood porphyria sufferers. Panhemin is also a drug used today to treat porphyria by limiting the liver’s production of porphyrins (Figure 6.27). Both treatments are derived from human 6 blood and have risks of carrying infectious agents. 8 9 0 Are Products of Biotechnology Helpful B or Harmful to Society? U Many products are made available through the use of biotechnology. Transgenic crops, for example have greater resistance to herbicides, and viral and fungal diseases. They are modified to withstand cooler temperatures longer and grow faster with larger fruits and vegetables. Soybeans, corn, cottonseed, and canola crops have seen large increases in transgenic numbers in the past decade, as shown in Figure 6.28. Over 93% of all soybeans and cotton crops are genetically modified in the United States. Eighty six percent of all corn, a major staple for cattle and humans, is produced by GMO organisms. If these organisms were not permitted to contribute to our food supply, would we be able to sustain our need to produce food, as a world population? ch06.indd 224 11/12/15 4:32 pm Chapter 6: Inheriting Genes 225 There have been big increases in farm production since the development of GMO foods. Crops are hardier and more productive, but it is a hotly debated area of study. The greater abundance of food means that fewer people go hungry. However, some GMO foods may also be linked to disease. A 2012 study in Europe shows that a corn variety, NK603 containing genes making it more resistant to the weed killer Roundup, was shown linked to cancer-causing effects when fed to a group of mice. Owing to this “cancer corn,” some European nations are placing restrictions on transgenic products. Is this fear of NK603 corn justified? The public has been consuming GMO products for over 15 years. No known ill effects have been confirmed by the scientific community in this time. What effects will be shown in 10, 20, or 30 years from now, is yet to be determined? Long-term results are not available because GMOs have not been S around long enough. M I T H The Things We’ve Handed Down: Should , We Tamper With Our Genes? HGH produced by gene manipulation for the past 25 years J helps extreme cases of growth disorders. Before recombinant DNA techniques were available, HGH was extracted from the pituitary glands of cadavers and carried theOrisk of contaminating patients. HGH is now fast and easy to produce, without contamination risks, making it more S commercially available. H This brings ethical and practical medical questions into play: Should a preteen male, U What if it is against the docpredicted to grow to a height of about 5' 4", take the drug? tor’s advice, which is based on the American Medical Associations guidelines to restrict A the drug only for extreme cases? What is an extreme case? What are the side effects? These are difficult questions to answer. 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 0 20 40 60 80 100 120 140 160 180 © Kendall Hunt Publishing Company 6 8 9 0 B U Figure 6.28 Graph showing relative increase in transgenic crops over the past 20 years. ch06.indd 225 11/12/15 4:32 pm 226 Unit 2: Is it all in the Genes? HGH has known side effects – from abnormal growth of joints to chronic pain, disfigurement and even death. HGH is used more and more in society to help teens reach a desired height. Some people are very happy with the results and other are devastated. There is uncertainty in medical procedures and treatments and their risks and benefits should be weighed. What are the social issues involved in being short? How many female readers would date a person taller than they are? I presume many would. How many would date a person shorter than they are? I am not sure. How many male readers would date a person shorter than they are? I presume most. How many male readers would date a person taller than they are? I am not sure. The reader should weigh the pros and cons of using technologies that scientists have made available to them. It is difficult to judge one another’s decision without understanding the social implications of the medical treatment. Ethically, will another doctor help the patient if one doctor denies treatment? What S is ethical for doctors to do if a patient desperately wants treatment to grow taller? These M are just a few of the provocative questions about HGH that some young Americans face every day. I T H , Myth, Fable, and Superstition Have Explained Maladies Incorrectly In the story at the beginningJ of the chapter, the character ponders his identity as a vampire. Throughout O history, unexplainable illnesses have often been linked with folklore of the occult, witchcraft, and creepy creatures. Epidemics of the plague, consumption S (tuberculosis), and the like often led to exhuming H victims to some sort of myth or fable based bodies and labeling one or more upon fear. All of these illnesses U had biological origins, as shown in our opening story, which shows the power of myth in shaping a person’s mental health and ­outlook—Was the narrator A in the story a victim of his own superstitions? 6 8 Summary 9 Heredity is the study of inheritance 0 of characteristics from parent to offspring. Predicted patterns of inheritance were discovered by Gregor Mendel in the 1800s. Mendel’s three laws describe inheritance of overB9,000 human traits. Inheritance of genetic information is more complex than Mendel U hypothesized. Genes interact with each other, the e­ nvironment, and sometimes share in their expression. Sexual reproduction results in a great deal of variation in populations. Meiosis, the forming of sex cells, produces unlimited genetic combinations within gametes. The flow of genes from one group to another is studied by population genetics. The numbers of different genotypes and phenotypes in a population are given by using the Hardy–Weinberg equation, with certain assumptions accepted. Biotechnology’s important component, gene technology, has resulted in many products available for public use. Gene technology products are continually being ­developed. Their effects on society and science continue to be debated as well. ch06.indd 226 11/12/15 4:32 pm Chapter 6: Inheriting Genes 227 Check Out Summary: Key Points • Heredity affects our physical characteristics, our environment and our future generations. • The discovery of inheritance by Gregor Mendel explains many of life’s characteristics. • Inheritance can be explained in future generations by probability using Punnett squares and in ­populations using the Hardy–Weinberg equation. • The stages and products of meiosis explain how sexual reproduction leads to great genetic variation. • Many traits in organisms are non-Mendelian, explained by codominance, polygenic inheritance, ­multiple alleles, and pleiotropy. • Pedigrees clarify gene flow within families. • Population genetics studies gene flow between and within populations. • Biotechnology has advances to provide productsS for human use, with debatable effects. Key Terms alleles anaphase I, II asexuality autosomal dominant autosomal recessive biotechnology crossing over cytokinesis diploid (2N) dominant fertilization gametes gene technology gene therapy genetic engineering genetically modified organism (GMO) haploid (N) heredity heterozygous homologous homozygous hypersexuality incomplete dominance kin selection law of dominance law of independent assortment ch06.indd 227 M I T H , law of segregation meiosis, meiosis I, meiosis II J characteristic, single-gene Mendelian trait O metaphase I, II S cross monohybrid multipleHalleles neurotransmitter U pedigree A pleiotropy polygenic traits population 6 genetics porphyria 8 I, II prophase recessive 9 recombinant 0 DNA technology sex chromosome B sex-linked somaticU cells telophase I, II testcross universal donor universal recipient X chromosome Y chromosome zygote 11/12/15 4:32 pm S M I T H , J O S H U A 6 8 9 0 B U ch06.indd 228 11/12/15 4:32 pm Chapter 6: Inheriting Genes 229 Multiple Choice Questions 1. Which is an inherited disorder? a. porphyria b. obesity c. Huntington’s disease d. all of the above 2. Which of Mendel’s laws was derived from the presence of 100% yellow phenotypes in the F1 generation? a. law of dominance b. law of independent assortment c. law of continuity S d. law of segregation M 3. The way in which an organism appears is its a. genotype b. phenotype c. pleiotropy d. codominance I T H , 4. If two heterozygous parents mate both carriers forJa recessively inherited form of porphyria) what is the chance that their offspring will have porphyria? O a. 0 S b. 25 c. 50 H d. 100 5. U Which stage of meiosis involves the separation of A homologous chromosomes? a. b. c. d. 6. anaphase I anaphase II prophase I prophase II 6 8 9 Which represents the correct flow of stages in meiosis? 0 a. prophase II➔metaphase I➔anaphase I➔telophase I B I b. prophase I➔metaphase I➔anaphase I➔telophase U II c. anaphase II➔prophase II➔telophase II➔metaphase d. telophase I➔anaphase I➔metaphase I➔prophase I ch06.indd 229 11/12/15 4:32 pm 230 Unit 2: Is it all in the Genes? 7. Which is the source of hemophilia for Prince Frederick using the pedigree in the figure below? a. grandmother b. grandfather c. mother d. uncle (a) S M I T H , (b) 8. In the Hardy–Weinberg equation, if the frequency of recessive alleles is 5% of the population, what is the number of recessive individuals in that population? J a. 25 out of 10 O b. 25 out of 100 S c. 25 out of 1,000 d. 25 out of 10,000 H 9. In question #8 above, what isUthe frequency of dominant genes in the population? A a. 5% b. 25% c. 75% 6 d. 95% 10. 8 Which statement best describes 9 the benefits of GMOs to society? a. Photosynthesis decreases0greenhouse gas effects. b. The food supply can support the population. B in check. c. Nonnative species are kept d. GMOs kill many speciesUof insects. Short Answers 1. Describe how porphyria affects the health of those inheriting it. Describe the ­mechanism by which porphyria causes damage. How does porphyria get portrayed as vampirism in history? Is it justified? Why or why not? ch06.indd 230 11/12/15 4:32 pm Chapter 6: Inheriting Genes 231 2. Define the following terms: phenotype, genotype, and pleiotropy. List one way each of the terms differ from each other in relation to heredity. Give an example found within fowl to make this clarification. 3. Describe the experiments of Gregor Mendel leading to the law of independent assortment. How does this law relate to genetic diversity within offspring? 4. In question #3, describe two other mechanisms by which genetic diversity is increased in populations through sexual reproduction. S 5. M I T Draw a Punnett square for a cross between two heterozygous tongue rollers? What H percentage of their offspring are heterozygotes? Homozygous dominant? , J 6. List the stages of meiosis I and II, indicating the point at which a cell becomes O ­haploid. Why does it become haploid at this point? 7. S H U What is an advantage of a Describe a testcross used to determine genotype in a pedigree. A testcross to determine genotype in a pedigree? Are its results certain? Why or why not? 8. 6 8 9 cystic fibrosis. What is the perOne in 22 people in the United States are carriers for centage of individuals who actually have this disease,0using the Hardy–Weinberg equation? Show your work. B U 9. Describe the process of recombinant DNA technology. Use the following terms to write its description: restriction enzyme, bacterial plasmid, vector, human DNA, protein. 10. Define the process of inbreeding. What are disadvantages of inbreeding? Are there any advantages? Explain your answers genetically. ch06.indd 231 11/12/15 4:32 pm 232 Unit 2: Is it all in the Genes? Biology and Society Corner: Discussion Questions 1. Diseases such a porphyria manifest in ways that give them a bad reputation. What other inherited traits have a bad reputation in society? Choose one trait and discuss how it is treated by the dominant culture. How are people with the trait treated differently? Suggest ways to improve the lives of people with this trait within our society. 2. The genetic basis of sexual preference was advocated for in this chapter. With which side do you agree, genetic or environmental in cause? What factors do you think limit or enhance the acceptance of alternative sexual preferences in society? Does the idea of a genetic basis have an impact in this acceptance? 3. If a society decided to remove all of the harmful recessive genes, such as cystic fibrosis, within its population, what would be its ethical difficulty? What would be its practical difficulty, basedS on the Hardy–Weinberg equation? Explain you answer fully. M 4. Race is used in decision-making regularly in the U.S. organizations. Why is race I such an important factor in society? Do you think it should be so? Is there a genetic basis to human race classifications? T 5. A health food guru claims that HGMOs are making people fat. Explain why this statement is false. How have GMO foods helped society? How have GMO foods harmed , Why or why not? society? Is your answer certain? J O S H U A 6 8 9 0 B U Figure – Concept Map of Chapter 6 Big Ideas ch06.indd 232 11/12/15 4:32 pm
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