Question

Your classmate told you that just by looking at numbers of progeny from a cross tell you if there is epistasis involved. They then give the following example:

In sheep, there are several different coat colors, a brown coat, an off-white coat, and a tan coat which is in between brown and off-white. Brown sheep crossed to brown sheep always yield only brown sheep. Off-white sheep crossed to off-white sheep yield only off-white sheep. Tan sheep crossed with tan sheep always yield a mix of brown, tan, and off-white sheep.

Two sheep with tan coats are crossed and their offspring are as follows: 23 tan coat, 10 brown coat, 9 off-white coat. But you know their example is wrong. Write a response to your classmate explaining how to determine what type of genetic inheritance is occurring and why. Be sure to reference all calculations and ratios associated with the progeny necessary to answer the question.

Answer 1

The above example is not epistasis. It is example of incomplete dominance. As the number of offspring shown in the example tan coat color is organisms is 23, brown 10 and off white 9 offspring. The number approximately gives 1:2:1 ratio of phenotype for brown, tan and off-white. Such inheritance is seen in incomplete dominance.

So, this cross can't be given as an example for epistasis. Here , neither brown allele (B) not white allele (W) are dominant. When both alleles are together it gives an intermediate trait which is tan(BW) coat color. Here the single gene is involved in the inheritance of 3 phenotypes. Epstasis is when genes of two different loci interact to produce a phenotype. Ratio of the epistasis is different from what we see in this cross. Dominant epistasis gives 12:3:1 ratio. Recessive epistasis gives 9:3:4 ratio. So ratio of the offsprimg is more closer to 1:2:1 ratio. SO it is incomplete dominance.