S4 - S1.Coat color in rodents is determined by a gene...

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S1.Coat color in rodents is determined by a gene interaction between two genes. If a true-breeding black rat is crossed to a true-breeding albino rat, the result is a rat with agouti (brownish/dark gray) coat color. If two agouti animals of the F 1 generation are crossed to each other, they produce agouti, black, and albino animals in a 9:3:4 ratio. Explain the pattern of inheritance for this trait. Answer: Since the parental generation is true-breeding, we know that the F 1 offspring are heterozygous for two genes. One strategy for solving this problem is to consider how a 9:3:4 ratio deviates from the 9:3:3:1 ratio we have already encountered in (two-gene) independent assortment problems. A 9:3:3:1 ratio has four categories of phenotypes. If we consider that the last two categories are combined via a gene interaction, this would yield a 9:3:4 ratio. With this idea in mind, we can then proceed to fill in a Punnett square to deduce the pattern of inheritance. The F 1 offspring are heterozygous for two genes. Let’s call them A (for albino) and C (for colored). In this case, C is dominant to c, and A is dominant to a. If an animal has at least one copy of both dominant alleles, it will have the agouti coat color. If an animal has a dominant A allele but is cc homozygous, it will develop a black coat. The four cases of albino animals all are aa homozygous. This occurs even when an animal carries the dominant C allele. Therefore, a is epistatic to C. The converse, however, does not yield the same result. Animals with ccAA or ccAa genotypes are black.
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S4 - S1.Coat color in rodents is determined by a gene...

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