Question

Erma and Harvey were a compatible barnyard pair, but a curious sight. Harvey�s tail was only 6 cm while Erma�s was 30 cm. Their F1 piglet offspring all grew tails that were 18 cm. When inbred (F1X F1), an F2 generation resulted in many piglets (Erma and Harvey�s grandpigs) whose tails ranged in 4 cm intervals from 6 to 30 cm (6, 10, 14, 18, 22, 26, 30). Most had 18 cm tails while 1/64 had 6 cm and 1/64 had 30 cm tails.a) Explain how tail length is inherited by describing the mode of inheritance, indicating how many gene pairs are at work, and designating the genotypes of Harvey, Erma, and their 18 cm offspring.b) If one of the 18 cm F1 pigs were mated with the 6 cm F2 pigs, what phenotypic ratio would be predicted if many offspring resulted?

Answer 1

a) Tail length shows quantitative mode of inheritance. It is controlled by three pair of genes and each allele contributes a small effect. Length of tail varies according to the cumulative effect shown by all the alleles present. Let the three genes controlling the trait be A,B and C. Dominant allele contributes 5 cm of tail length whereas recessive allele contributes 1 cm of tail length.

Harvey = aabbcc = 1*6 = 6 cm

Erma = AABBCC = 5*6 = 30 cm

When Harvey and Erma mate : AABBCC X aabbbcc : AaBbCc (F1)

AaBbCc = (3*5) + (3*1) = 18 cm

b) AaBbCc ( 18 cm ) X aabbcc ( 6 cm ) = 8 type of offspring will be produced in 1:1:1:1:1:1:1:1 ratio :

abcabc, Abcabc, aBcabc, abCabc, ABcabc, aBCabc, AbCabc, ABCabc