Genetics
1. In humans, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a blue-eyed woman and they have four children, some whom are brown-eyed and some of whom are blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s genotype? What are the genotypes of the children?
2. In dogs, there is a hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either homozygous or heterozygous. If the dog’s genotype is heterozygous, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female. Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage/how many of the offspring would be expected to be hearing? deaf? How could you tell the genotype of this male dog? Also, using Punnett square(s), show how two hearing dogs could produce deaf offspring
3. In pea plants, yellow peas are dominant over green peas. Use a Punnett square to predict the phenotypic and genotypic outcome (offspring) of a cross between two plants heterozygous for yellow peas. What is the genotypic and phenotypic ratio?
4. In radishes, an incompletely dominant gene controls color. Red and white radishes are homozygous, whereas the heterozygous is purple. Complete a punnet square to show the genotypes and phenotypes produced by crossing pure breeding red radishes with white radishes.
5. Some genes have more than two alleles. One of the best-known examples is the gene that is referred to as the “ABO Blood Group,” which actually has quite a number of alleles. However, we will discuss/consider only the three most-common of these. This gene codes for the structure of a certain antigen on the surface of our RBCs. The three alleles we will work with are symbolized by IA, IB, and i. However, keep in mind that a person can only have two alleles, two copies of a gene. Thus, the possible genotypes are IAIA, IAi, IBIB, IBi, IAIB, or ii.
Suppose a person with type A blood and a person with type B blood get married. What are all of the possible genotypes and phenotypes their children could have?
Suppose a person with type A blood and a person with type O blood get married. What are all of the possible genotypes and phenotypes their children could have?
6. In humans, the genes for colorblindness and hemophilia are both located on the X chromosome with no corresponding gene on the Y. These are both recessive alleles. If a man and a woman, both with normal vision, marry and have a colorblind son, draw the Punnett square that illustrates this. If the man dies and the woman remarries to a colorblind man, draw a Punnett square showing the type(s) of children could be expected from her second marriage. How many/what percentage of each could be expected?
7. Let’s say that in seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers.
a) What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous? What is the genotypic and phenotypic ratio?
b) If one parent seal is pure long-whiskered and the other is short-whiskered, what percent of offspring would have short whiskers?
8. In purple people eaters, one-horn is dominant, and no horns is recessive. Draw a Punnet Square showing the cross of a purple people eater that is hybrid for horns with a purple people eater that does not have horns. What is the genotypic and phenotypic ratio?
9. A woman with type A blood has a child with type O blood. She is suing a man with type B blood for child support, because she claims that man is the father of her child. How would you respond to the following statements?
- The attorney for the alleged father claims “The mother’s blood is type A, so the child’s type O blood must have come from the father. Because my client has type B blood, he can not be the father.” Justify your answer with appropriate Punnett-square(s) or receive 0 points!
- The attorney for the mother claims “Because further tests prove he is heterozygous, he must be the father.”
10. Colorblindness is inherited as a sex-linked recessive disease. An affected male marries a heterozygous female. Draw a Punnett square of the possible offspring. What is the genotypic and phenotypic ratio? What is the chance that they will have an affected child? Could any of their daughters be affected?
