Question

Alleles at two gene loci influence % protein in pea leaves. Each locus assorts independently. There are two alleles at each locus; alleles A and a at locus 1, and alleles B and b at locus 2. Each upper-case allele (e.g. “A”) has an additive and cumulative effect that increases the % protein by the same amount (20%) for each upper-case allele contained in an individual. The upper-case alleles are the “contributing” alleles. There are no interlocus interactions and no environmental effects.

Plants with genotype AA BB have 90% protein. Plants with genotype aa bb have 10% protein (so 10% is the baseline and minimum protein content they can have).

Plant 1, 90% protein, is mated with Plant 2, 10% protein, produce the F1 generation.    The F1-generation plants are mated with each other to produce the F2 generation.

a. What is the expected % protein of the F1-generation plants?

b. How many phenotypic classes are expected in the F2 generation?

c. Which genotypes will be true-breeding for 50% protein in the F2 generation?

d. What fraction of the F2 generation is expected to have a phenotype of 50% protein?

Answer 1

a. What is the expected % protein of the F1-generation plants?
The answer is 50%. Becuase the F1 has two dominant genes, so, it addes 40% to the basic level.

b. How many phenotypic classes are expected in the F2 generation?
The answer is 5. Find the below image for explanation. Those are with 90, 70%, 50%, 30% and 10%.

c. Which genotypes will be true-breeding for 50% protein in the F2 generation?
The answer is AAbb, aaBB. Find the below image for explanation.

d. What fraction of the F2 generation is expected to have a phenotype of 50% protein?
The answer is 6/16. Find the below image for explanation.