Sometimes one gene can affect the expression of another gene (epistasis ). A good example of epistasis is the genetic interactions that produce coat color in horses and other mammals. In horses, brown coat color (B ) is dominant over tan (b ). However, how that gene is expressed in the phenotype is dependent on a second gene that controls the deposition of pigment in hair. The dominant gene (C ) codes for the presence of pigment in hair, whereas the recessive gene (c ) codes for the absence of pigment. If a horse is homozygous recessive for the second gene (cc ), it will have a white coat regardless of its coat color genotype (B gene) because pigment is not deposited in the hair. This figure shows a Punnett square for this example. The white horses have a genotype specifying brown or tan coat color at the first gene, but are completely white because they are homozygous recessive for the gene controlling pigment deposition.At the following Web sites, find the correct answer to the multiple-choice dihybrid cross questions. Work out each problem yourself using paper and pencil. To view an explanation of the problem, select the "TUTORIAL" button. After viewing the correct answer, close the Dihybrid Cross Problem Set window to return to this page. (These sites are a part of the Dihybrid Problem Sets provided by The Biology Project at the University of Arizona.)
Problem 12 : What is the genotype of the agouti parent? - This problem is a part of the Dihybrid Cross Problem Set.
Problem 13 : AaBb dihybrid cross involving epistasis - This problem is also a part of the Dihybrid Cross Problem Set.
Note that epistasis is distinct from polygenic inheritance. Epistasis is an interaction that occurs when one gene directly affects the expression of another, while polygenic inheritance is the total influence of several genes on a single trait. While distinct, the terms are also not mutually exclusive; some polygenic inheritances may also show epistatic interactions.