Question

1. Assume that tail length in white rats is a quantitative trait. In one pure-breeding line the tail averages 7 inches and in another it averages 4 inches. Progeny arising from the interbreeding of these two lines have tails which average 5.5 inches. When members of the F1 interbreed, the 496 offspring have tails which also average 5.5 inches in length, but vary much more than those of their parents, with the range between 4 and 7 inches. Two members of the F2 have 7-inch tails.

1. Based on these data, what can be concluded about the number of pairs of genes involved in determining tail length?
2. How many members of the F2 would be expected to have four-inch tails?
3. Assuming an equal contribution from each effective allele, how much does each allele contribute to the trait?

Answer 1

aabbccdd = 4 inches
AABBCCDD = 7 inches

It is given that two individuals out of 496 have a 7 inch tail
This is approximately, one in 256
4^4 = 256
= The trait must be regulated by 4 genes (Each gene with 2 alleles)
Contribution from each allele = 0.375 inches

Parental cross: AABBCCDD (7 in) X aabbccdd (4 in)
F1 progeny: AaBbCc (5.5 in)

a. The number of pair of genes = 4 genes with 2 alleles each

b. If the number of individuals with 7 inches tail length = 2,
The number of individuals with 4 inches tail length = 2

c. Contribution from each allele = 0.375 inches