Inheritance Name ________________________________ Simple inheritance is the phenotypic results of the impacts of a single gene. In Simple Dominance, one allele is dominant and the other is recessive. When an individual is heterozygous (having one dominant allele and one recessive allele), ONLY the dominant allele is expressed in the phenotype. NOTE: In this lab, we will focus on traits for which only two alleles exist. In these cases, it is least confusing to refer to them by the same letter. Generally, the dominant allele is written as a capital letter, and the recessive allele is lower case. For letters where the upper and lower case letters are very similar, the lower case letter may be underlines (for example: M and m). Consequently, a pea plant’s genotype for this trait may be MM, Mm, or mm. If we wish to compare multiple genes, the pea plant’s genotype will be a string of paired letters. For example MMTtyy might be the genotype for the M, T, and Y genes. Example: In Mendel’s pea plants, purple flower color is dominant over white flower color (white is then called recessive). We will call the purple allele P and the white allele p. A flower with the genotype PP would be purple, since it has only purple alleles. Similarly, a flower with the genotype pp would be white, since it has only white alleles. A flower that has the genotype Pp has one purple allele and one white allele, but since purple is dominant, the white color is not expressed and the flower will be purple. Part One: Practicing Terminology 1. In a simple dominant trait with two alleles (P and p), list all possible genotypes along with the corresponding phenotypes (dominant or recessive). 2. What is the difference between heterozygous and homozygous? 3. Circle/highlight the homozygous dominant genotype: PP Pp pp 4. If you know purple flower color is dominant over white flower color, is a white flower heterozygous or homozygous? Explain your answer. 5. If you know purple flower color is dominant over white flower color, is a purple flower heterozygous or homozygous? Explain your answer. Part Two: Simple Inheritance Problems 1 When discussing inheritance, it is helpful to first start with the gametes that a parent can produce to create its offspring. In the example on page one, the purple flower with the genotype PP can only produce gametes that carry the P allele, since that is the only allele it carries. Similarly, the white flower can only produce gametes that carry the p allele, since that is the only allele it carries. The heterozygous purple flower (genotype Pp) can produce gametes carrying P or p since it carries both alleles. Many of these questions will involve the use of Punnett Squares. Use the following steps to work through a Punnett Square: • • • • • Step one: determine what gametes each parent can produce Step two: draw a Punnett Square and write the possible gametes for one parent across the top, and for the other parent along the left side Step three: fill in the Punnett Square with the possible combinations of gametes the offspring could receive Step four: determine the phenotype that each genotype exhibits Step five: sometimes, you will also need to calculate the fraction or percentage of offspring that exhibit a particular genotype or phenotype Example Problem: In pea plants, purple flower color (allele F) is dominant over white flower color (allele f). If a heterozygous purple flower mates with a white flower, what percentage of the offspring would you expect to be purple? What percentage do you expect to be white? Step one: what gametes can each parent produce? The purple flower is genotype Ff: the gametes it can produce are F and f The white flower is genotype ff. the gamete it can produce is only f Step two, three, and four: draw and complete a Punnett Square and note the corresponding phenotype for each offspring genotype (purple) Step five: What percentage of offspring would you expect to be purple? 100% What percentage of offspring would you expect to be white? 0% 6. The Punnett square is used to figure out all possible results of mating between individuals. In this way, we examine the gametes in all possible combinations. We know purple flower color (allele F) is dominant to white (allele f). a. complete a Punnett square for a cross between a heterozygous purple-flowered plant and a white-flowered plant. b. What are the possible genotypes of the offspring of this cross? c. What are the possible phenotypes of the offspring of this cross? d. What percentage of the offspring from this cross would you expect to be homozygous? e. What percentage of the offspring from this cross would you expect to exhibit the recessive phenotype? 7. For each example below, complete a Punnett Square for the cross described. If letters denoting the allele are not provided, choose your own letters to represent each allele. Make sure to provide the genotypes and phenotypes of the offspring. a. A tall plant (TT) is crossed with a short plant (tt) b. Two hetrozygous brown mice are crossed 8. List the gametes that can be produced by a pea plant with the following genotypes. Remember: an allele for every gene considered must be present in each gamete a. Pp b. TT c. PpTt d. PpTT e. PPTtYY f. PpTtYy 9. In tomatoes, the texture of the skin may be smooth (dominant) or ‘hairy’ (recessive, like a peach). The Ponderosa variety has fruits with smooth texture, while the Red Peach variety has fruits with hairy texture. Crosses between the two varieties produce all smooth fruits. a. What percentage of the cross offspring are smooth? b. What can you say about the genotypes of the initial parents? 10. A brown mink is crossed with a silverblue mink, and all the offspring are brown. When these F1 brown minks mate, they produce the F2 generation which consists of 47 brown offspring and 15 silverblue offspring. Determine the genotype(s) of: a. the original brown mink: b. the original silverblue mink: c. the F1 generation: d. the F2 generation: e. How many of the F2 individuals would be homozygous? 11. In horses, black coat color is dominant, with chestnut color as the recessive phenotype. The trotting gait is also dominant, with pacing as the recessive phenotype. a. If a homozygous black pacer is mated to a homozygous chestnut trotter, what will be the phenotype of the F1 generation? b. If an F1 male from this cross is mated to a homozygous female black pacer, what will be the phenotypes of the offspring, and in what proportions? Part Four: Codominance and Incomplete Dominance Regardless of the type of dominance, a homozygote will express the phenotype of a single allele. Heterozygotes, however, can be more complicated. Last week’s exercises focused on simple dominance, in which the phenotype of the heterozygote follows the dominant allele (and is therefore identical to the homozygous dominant phenotype). There are two other forms of dominance, however; incomplete dominance in which the heterozygote phenotype is a blend of the dominant and recessive phenotype, and codominance, in which the heterozygote expresses both the dominant and recessive phenotypes separately. For example: Imagine a red flower with the genotype RR and white flower with the genotype rr are crossed. Some of the offspring will have the genotype RR and be red, others will have the genotype rr and be white regardless of the type of dominance. If the alleles show simple dominance, the heterozygote (genotype Rr) will be red (the dominant phenotype). If, however, the alleles show incomplete dominance, the heterozygote will be pink. And if the alleles are codominant, the flower will be red and white, showing stripes or spots of both colors (not blended into pink). 12. In a certain type of flower, the dominant color is blue (B) and the recessive color is red (b). Two heterozygote blue flowers are crossed. a. What are the potential genotypes of this cross? b. What are the potential phenotypes of this cross if this trait exhibits simple dominance? c. What are the potential phenotypes of this cross if this trait exhibits co-dominance? d. What are the potential phenotypes of this cross if this trait exhibits incomplete dominance? 13. When more than two alleles exist, multiple types of dominance may occur within a single trait. Blood type in humans is a good example of this. The A allele exhibits simple dominance over the O allele. The B allele also exhibits simple dominance over the O allele. The A and B alleles, however, exhibit codominance with each other. In addition, there are two alleles for the Rh factor in blood. The Rh-positive allele (+) exhibits simple dominance over the Rh-negative allele (-). In the table below, fill in the phenotype (blood type) for each genotype. For example, the genotype BO+/- has one allele for the B blood type and one for the O blood type, one allele for Rh-positive and one allele for Rh-negative. The B allele is simple dominant and the Rh-positive allele is simple dominant, meaning the phenotype for this genotype is blood type B-positive. Complete the following table by filling in the phenotypes which match the given genotypes. Genotype AA+/+ AB+/- AB+/+ OO-/- AO+/- BO-/- BB+/- AB-/- Phenotype 14. Two parents believe their baby was switched at the hospital with someone else’s child. The mother has blood type O+, the father has blood type AB+, and the baby has blood type B-. a. What are the possible genotypes of the parents and the baby? b. What are the possible gametes that can be produced by the parents? Mother: Father: c. Is the baby theirs? Explain your answer using Punnet Square(s). Part Five: Multiple Gene Interaction In some cases, an organism’s appearance is controlled by more than one gene. In these cases, the various genotypes interact to produce a phenotype. 15. Coat color in Labrador Retrievers is controlled by two separate genetic loci. The first controls the color of pigment and is called the ‘brown’ locus. The dominant allele (B) produces black labs, while recessive alleles (b) produces chocolate labs. This trait exhibits simple dominance, so heterozygote labs (Bb) are also black. The second locus controls whether or not pigments are expressed in only the skin, or also in the fur, termed the ‘extension’ trait. This trait also exhibits simple dominance, so dogs with genotype EE or Ee will exhibit whatever coat color is indicated by the brown locus. Dogs that are homozygous recessive (ee), however, will exhibit no pigmentation in their fur and will be yellow labs regardless of their genotype at the brown locus. The relationship is shown below: a. List the nine possible genotypes of a Labrador Retriever and the phenotype associated with that genotype. b. Two chocolate labs produce a litter of puppies, one of which is a yellow lab. What is the genotype of the yellow lab pup? What are the genotypes of the parents? c. This yellow lab puppy (who happens to be male) later grows up and befriends a lady yellow lab down the street. The lady yellow lab produces a litter of puppies, which are a mix of chocolate and black. The owner of the lady yellow lab claims our male yellow lab sired the puppies. Is the owner correct? Why or why not? d. Our male yellow lab befriends another lady, this time a black lab. The owner of this lady black lab actually observes the interaction (which is a little awkward for everyone involved), so we know our male yellow lab is the sire. This lady black lab later produces a litter of puppies that are a mix of black, chocolate, and yellow. What is the genotype of the mother of the puppies? e. What are the genotypes of the puppies produced in this cross? 
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