Genetics Practice Problems

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Monohybrid Cross:

In humans, brown eyes (B) are dominant over blue (b)*. A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the mans genotype? What are the genotypes of the children?

(* Actually, the situation is complicated by the fact that there is more than one gene involved in eye color, but for this example, well consider only this one gene.)

Testcross:

In dogs, there is an 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 DD or Dd. If the dogs genotype is Dd, 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 (dd). 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.

Incomplete Dominance:

Note: at least one textbook Ive seen also uses this as an example of pleiotropy (one gene multiple effects), though to my mind, the malaria part of this is not a direct effect of the gene.

For many genes, such as the two mentioned above, the dominant allele codes for the presence of some characteristic (like, B codes for make brown pigment in someones eyes), and the recessive allele codes for something along the lines of, I dont know how to make that, (like b codes for the absence of brown pigment in someones eyes, so by default, the eyes turn out blue). If someone is a heterozygote (Bb), that person has one set of instructions for make brown and one set of instructions for, I dont know how to make brown, with the result that the person ends up with brown eyes. There are, however, some genes where both alleles code for something. One classic example is that in many flowering plants such as roses, snapdragons, and hibiscus, there is a gene for flower color with two alleles: red and white. However, in that case, white is not merely the absence of red, but that allele actually codes for, make white pigment. Thus the flowers on a plant that is heterozygous have two sets of instructions: make red, and make white, with the result that the flowers turn out mid-way in between; theyre pink.

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Genetics Practice Problems - University of Cincinnati

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