Assume that three independently segregating loci each with t

Assume that three independently segregating loci each with two alleles (A+ and A-, B+ and B-, C+ and C-) determine height in a plant. These genes are additive with each plus allele adding 3 cm to a base height of 3 cm.

a) What are the heights expected in the F1 progeny of a cross between homozygous strains:

A+A+B+B+C+C+ (21cm) x A-A-B-B-C-C- (3cm)?

b) What proportion of F2 plants will have heights equal to the heights of the orginal two parental strains:

A+A+B+B+C+C+ and A-A-B-B-C-C-?

c) What is the distribution of heights (frequency and phenotype) expected in an F1 X F1 cross?

Solution

a) Since the cross is between a homozygous dominant and a homozygous recessive, all the progeny will have the genotype A+A-B+B-C+C-, and phenotypically, they will all be of a height of 3(height added by each plus allele) x 3(number of plus alleles) + 3(base height) = (3x3)+3 = 12 cm.

b) Out of the F2 plants(64 progeny), only 2 plants will have heights equal to the original parents. The proportion will be 1 in 64 for both.

c) Out of the 64 progeny obtained from F1 X F1 cross, 1 will be 21cm, 6 will be 18cm, 13 will be 15cm, 20 will be 12cm, 17 will be 9cm, 6 will be 6cm and 1 will be 3cm in height. The ratio would be 1:6:13:20:17:6:1.