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Mendelian Patterns of Inheritance Fundamentals of Genetics • Genetics is the branch of science that studies how the characteristics of living organisms are inherited. • An allele is a specific version of a gene. • Examples: eye color, hair color, earlobe type • The two different alleles are on the same part of a chromosome. Fundamentals of Genetics • Gene: A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism. • An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. • Locus: (plural loci) is the specific location of a gene or DNA sequence or position on a chromosome. Fundamentals of Genetics • The interaction of alleles determines the appearance of the organism. – The genotype of an organism is the combination of alleles that are present in an organism’s cells • Ex. BB, Bb, bb • Homozygous – two identical alleles • Heterozygous – two different alleles – The phenotype of an organism is how it appears outwardly and is a result of an organism’s genotype • Blue eyes, brown eyes Fundamentals of Genetics • A dominant allele masks the recessive allele in the phenotype of an organism – Dominant allele is usually shown by a capital letter – Recessive alleles are usually shown by a lower-case letter. • B – brown-eyes, b – blue-eyes • BB, Bb, bb. Fundamentals of Genetics • Genetic cross is a planned mating between two organisms • Punnett square shows the possible offspring of a particular genetic cross • AS * SS A S S AS SS S AS SS Punnett Square • Height: T=tall, t=short • Question The genotype of plant’s parent is below. Identify all the Genotypes and Phenotypes of the offspring using the Punnet square table. TT x tt Tt x Tt TT x Tt Mendel’s Laws of Genetics • Mendel recognized genetic principles • Organisms have two pieces of genetic information for each trait (later called alleles) • Law of Dominance states that some alleles interact with each other in a dominant and recessive manner, where the dominant allele masks the recessive trait. • Ex: PP = purple pp = white • Pp = purple • Law of Segregation says when a diploid organisms forms gametes, the two alleles for a characteristic separate from one another. One-Trait Inheritance • Mendel performed cross-breeding experiments • Used “true-breeding” (homozygous) plants • Chose varieties that differed in only one trait (monohybrid cross) • Performed reciprocal crosses • Parental generation = P • First filial generation offspring = F1 • Second filial generation offspring = F2 • Formulated the Law of Segregation Mendel’s Monohybrid Crosses: An Example Dihybrid crosses • Matings that involve parents that differ in two genes (two independent traits). For example, flower color: P = purple (dominant) p = white (recessive) and stem length: T = tall t = short Dihybrid cross: flower color and stem length (shortcut) TT PP  tt pp (tall, purple) Possible Gametes for parents (short, white) T P TP tp t p F1 Generation: All tall, purple flowers (Tt Pp) Tt Pp Dihybrid cross F2 If F1 generation is allowed to self pollinate, Mendel observed 4 phenotypes: Tt Pp  Tt Pp (tall, purple) (tall, purple) TP Possible gametes: TP Tp tP tp Tp tP tp Tp TTPP TTPp TtPP TTPp TTpp TtPp TtPp Ttpp tP TtPP TtPp ttPP ttPp TtPp Ttpp ttPp ttpp TP tp Four phenotypes observed Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1) Dihybrid cross 9 Tall purple TP TP 3 Tall white Tp tP tp 3 Short purple 1 Short white Tp tP tp TTPP TTPp TtPP TTPp TTpp TtPp TtPp Ttpp TtPP TtPp ttPP ttPp TtPp Ttpp ttPp ttpp Phenotype Ratio = 9:3:3:1 Law of Segregation • Each individual has a pair of factors (alleles) for each trait. • The factors (alleles) segregate (separate) during gamete (sperm & egg) formation. • Each gamete contains only one factor (allele) from each pair. Fertilization gives the offspring two factors for each trait. Sample Problems 1. A coined is flipped four times and comes up with a head each time. What is the probability that the next coin flip will come up with a head? 2. Classify the following as heterozygous or homozygous: RR, Rr, yy 3. What is the Phenotype of the following of the following: Yy, Rr, yy 4. What is the probability of Rr x Rr producing wrinkled seeds? 5. What is the probability of Yy x yy producing green seeds? Investigating Mendelian conditions in human A member of a family who first comes to the attention of a geneticist is called the proband. Usually the phenotype of the proband is exceptional in some way (for example, the proband might be a dwarf). Males are hemizygous for loci on the X and Y chromosomes, where they have only a single copy of each gene, so the question of dominance or recessiveness does not arise in males for X- or Y-linked characters. The five basic Mendelian pedigree patterns • Mendelian characters may be determined by loci on an autosome or on the X or Y sex chromosomes. Autosomal characters in both sexes and X-linked characters in females can be dominant or recessive. Thus there are five archetypal Mendelian pedigree patterns: 1. 2. 3. 4. 5. Autosomal dominant Autosomal recessive X-linked recessive X-linked dominant Y-linked • Only one important gene has been located on the human Y chromosome, the TDF gene, which codes for a testis-determining factor and plays a primary role in maleness. • Therefore, in practice the important mendelian pedigree patterns are autosomal dominant, autosomal recessive and X-linked. Autosomal Dominant Disorders • Autosomal dominant inheritance means that the gene carrying a mutation is located on one of the autosomes (chromosome pairs 1 through 22). This means that males and females are equally likely to inherit the mutation. "Dominant" means that having a mutation in just one of the two copies of a particular gene is all it takes for a person to have a trait, such as an increased risk of developing cancer A pedigree showing autosomal dominant inheritance Autosomal dominant pedigree pattern • In pedigree analysis, the main clues for identifying a dominant disorder are that the phenotype tends to appear in every generation of the pedigree and that affected fathers and mothers transmit the phenotype to both sons and daughters. • Examples of autosomal dominant disorders include Huntington's disease and neurofibromatosis-1. Pedigree of a dominant phenotype Autosomal Recessive Disorders • Autosomal recessive inheritance means that the gene is located on one of the autosomes (chromosome pairs 1 through 22). This means that males and females are equally affected. • "Recessive" means that two copies of the gene are necessary to have the trait, one inherited from the mother, and one from the father. • A person who has only one recessive gene is said to be a "carrier" for the trait or disease, but they do not have any health problems from "carrying" one copy of the gene. • Once parents have had a child with a recessive trait or disease, there is a one out of four, or 25 percent chance, with each subsequent pregnancy, for another child to be born with the same trait or disorder. Autosomal Recessive Pedigree for Sickle Cell disease Examples of autosomal recessive disorders include cystic fibrosis, sickle cell anemia. X-Linked Recessive Pedigree X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be expressed (1) in males (who are necessarily hemizygous for the gene mutation because they have only one X chromosome) and (2) in females who are homozygous for the gene mutation (i.e., they have a copy of the gene mutation on each of their two X chromosomes). red-green color blindness. X-Linked Dominant Pedigree X-linked dominant traits do not necessarily affect males more than females (unlike X-linked recessive traits). The exact pattern of inheritance varies, depending on whether the father or the mother has the trait of interest. All daughters of an affected father will also be affected but none of his sons will be affected (unless the mother is also affected). In addition, the mother of an affected son is also affected (but not necessarily the other way round)e.g. Rett Syndrome. Complete Dominance • In the genes that Mendel examined, one allele demonstrated complete dominance. • In heterozygotes, the dominant allele was expressed in the phenotype and the alternative allele (recessive) was repressed. • An individual with a dominant phenotype could have either a homozygous dominant genotype (PP) or a heterozygous genotype (Pp). Incomplete Dominance • Incomplete Dominance is a type of inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype (expressed physical trait). • For example, if you cross pollinate red and white snapdragon plants, the dominant allele that produces the red color is not completely dominant over the recessive allele that produces the white color. The resulting offspring are pink Codominance • In codominance the effects of both alleles are visible as distinct effects on the phenotype. • A good example of codominance is expression of the A and B blood type alleles in humans. • Multiple Alleles refers to situations in which there are more than 2 possible alleles that control a particular trait • For blood type there are three different alleles • IA – blood has type A antigen on rbc surface • IB – Blood has type B antigen on rbc surface • i – Blood type O has neither type A nor type B antigens on rbc surface • Interactions Among Alleles • For blood type there are three different alleles • IA – blood has type A antigen on rbc surface • IB – Blood has type B antigen on rbc surface • i – Blood type O has neither type A nor type B antigens on rbc surface • Type A blood has anti-B antibodies. • Type B blood has anti-A antibodies • Type O blood has no antibodies for A or B Codominance • Type O individuals (ii) are universal donors and type AB are universal recipients Polygenic Inheritance • Polygenic inheritance refers to the kind of inheritance in which the trait is produced from the cumulative effects of many genes in contrast to monogenic inheritance wherein the trait results from the expression of one gene (or one gene pair). In humans, height, weight, and skin color are examples of polygenic inheritance, which does not follow a Mendelian pattern of inheritance. Pleiotropy: Pleiotropy occurs when the alleles from a single gene have multiple phenotypic effects. THE END
Mitosis: Cell Division ‘’Dance of Chromosomes’’ Chromosome The Key Roles of Cell Division • The ability to reproduce is one of the key features that separates life from non-life. • All cells have the ability to reproduce, by making exact copies of themselves. • In unicellular organisms, division of one cell reproduces the entire organism • In multicellular organisms, cell division is needed for: • Development of an embryo from a sperm/egg • Growth • Repair Asexual Reproduction • Asexual reproduction is reproduction that involves a single parent producing an offspring. • The offspring produced are, in most cases, genetically identical to the single cell that produced them. • Prokaryotic organisms (like bacteria) reproduce asexually, as do some eukaryotes (like sponges). Also termed Binary fission. Prokaryotic Chromosomes • Prokaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm. • Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information. Eukaryotic Chromosomes • In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin. • Chromatin is composed of DNA and histone proteins. • DNA coils around histone proteins to form nucleosomes. • The nucleosomes interact with one another to form coils and supercoils that make up chromosomes. Sexual Reproduction • In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow. • The offspring produced inherit some genetic information from both parents. • Most animals and plants, and many single-celled organisms, reproduce sexually. Contrasting Reproduction types Cell Division • Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA. • A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells. • A cell’s endowment of DNA (its genetic information) is called its genome. • DNA molecules in a cell are packaged into chromosomes. Terminologies • DNA: is the hereditary material in humans and almost all other organisms. Most DNA is located in the cell nucleus (where it is called nuclear DNA. • GENE: A gene is a locus (or region) of DNA that encodes a functional RNA or protein product, and is the molecular unit of heredity. • CHROMOSOME: In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. •. Terminologies • CHROMATID: A chromatid is one copy of a newly replicated chromosome, which typically is joined to the other copy by a single centromere. Before replication, one chromosome is composed of one DNA (double-helix) molecule. • CHROMATIN: is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. • NUCLEOSOME: The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Terminologies • CENTROMERE: The centromere is the part of a chromosome that links sister chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore. • KINETOCHORE:A kinetochore is a protein structure that forms on a chromatid during cell division and allows it to attach to a spindle fiber on a chromosome. • HISTONES: are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, and playing a role in gene regulation. Chromosome Terminologies • TELOMERASE: is a ribonucleoprotein that adds the polynucleotide "TTAGGG" to the 3' end of telomeres, which are found at the ends of eukaryotic chromosomes. • TELOMERE: is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. • SEPARASE: also known as separin, is a cysteine protease responsible for triggering anaphase by hydrolyzing cohesin, which is the protein responsible for binding sister chromatids during the early stage of anaphase. Terminologies • COHESIN: Protein complexes that glues sister chromatids at the centromere. • CONDENSIN: is an elongated complex of several proteins that binds and encircles DNA. In contrast to cohesin, which binds two sister chromatids together, condensin binds a single chromatid at multiple spots, twisting the chromatin into a variety of coils and loops. • SEPARASE: also known as separin, is a cysteine protease responsible for triggering anaphase by hydrolyzing cohesin, which is the protein responsible for binding sister chromatids during the early stage of anaphase. Chromosomes • The genetic information that is passed on from one generation of cells to the next is carried by chromosomes. • Every cell must copy its genetic information before cell division begins. • Each daughter cell gets its own copy of that genetic information. • Cells of every organism have a specific number of chromosomes. Chromosome Chromosomes During Cell Division • In preparation for cell division, DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division • The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached Phases of the Cell Cycle • The cell cycle consists of • Interphase (cell growth and copying of chromosomes in preparation for cell division) • Mitotic (M) phase (mitosis and cytokinesis) • Interphase (about 90% of the cell cycle) can be divided into sub phases: • G1 phase (“first gap”) • S phase (“synthesis”) • G2 phase (“second gap”) G1 Phase: Cell Growth • In the G1 phase, cells increase in size and synthesize new proteins and organelles. S Phase: DNA Replication • In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated. G2 Phase: Preparing for Cell Division • In the G2 phase, many of the organelles and molecules required for cell division are produced. M Phase: Cell Division • In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis. • Mitosis is the division of the cell nucleus. • Cytokinesis is the division of the cytoplasm. Prophase/Prometaphase • During prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible. • The centrioles move to opposite sides of nucleus and help organize the spindle. • The spindle forms and DNA strands attach at a point called their centromere. • The spindle forms and DNA strands attach at a point called their centromere. • The nucleolus disappears and nuclear envelope breaks down. Prophase Prometaphase Metaphase • During metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell. The spindle fibers connect the centromere of each chromosome to the two poles of the spindle. • The spindle fibers connect the centromere of each chromosome to the two poles of the spindle. Anaphase • During anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes. • The chromosomes separate into two groups near the poles of the spindle. Telophase • During telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin. • A nuclear envelope re-forms around each cluster of chromosomes. • The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus. Cytokinesis • Cytokinesis is the division of the cytoplasm. • The process of cytokinesis is different in animal and plant cells. • The cell membrane is drawn in until the cytoplasm is pinched into two equal parts. • Each part contains its own nucleus and organelles. The Cell Cycle Control System • The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock • The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received. For many cells, the G1 checkpoint seems to be the most important one • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a non dividing state called the G0 phase Loss of Cell Cycle Controls in Cancer Cells • Cancer cells do not respond normally to the body’s control mechanisms • Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue • If abnormal cells remain at the original site, the lump is called a benign tumor • Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors Phase Interphase Prophase Chromosome Appearance & Location DNA copies itself; chromatin Chromosomes coil up Important Events DNA replication, cell grows and replicates organelles Nuclear envelope disappears, spindle fibers form Chromosomes line up in the middle Spindle fibers connect to chromosomes Anaphase Chromosome copies divide and move apart Spindle fibers pull chromosome copies apart to opposite poles Telophase Chromosomes uncoil back into chromatin Nuclear envelopes reform, 2 new nuclei are formed, spindle fibers disappear Metaphase Cytokinesis Chromatin Division of the rest of the cell: cytoplasm and organelles I.P.M.A.T INTERPHASE PROPHASE METAPHASE ANAPHASE TELOPHASE Apoptosis: Programmed Cell Death Apoptosis or programmed cell death, is carefully coordinated collapse of cell, protein degradation , DNA fragmentation followed by rapid engulfment of corpses by neighboring cells. EXAMPLES The formation of the fingers and toes of thefetus The sloughing off of the inner lining of the uterus The formation of the proper connections between neurons in the brain . Apoptosis is needed to destroy cells Examples: Cells infected with viruses Cells of the immune system Cells with DNA damage Cancer cells • THE END

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