Question

A dark-red strain and a white strain of wheat are crossed and produce an intermediate, medium-red F1. When the F1 plants are interbred, an F2 generation is produced in a ratio of 1 dark-red : 4 medium-dark-red : 6 medium-red : 4 light-red : 1 white Further crosses reveal that the dark-red and white F2 plants are true breeding.

a. Based on the ratios in the F2 population, how many genes are involved in the production of color?
b. How many additive alleles are needed to produce each possible phenotype?
c. Assign symbols to these alleles and list possible genotypes that give rise to the medium red and light red phenotypes.
d. Predict the outcome of the F1 and F2 generations in a cross between a true-breeding medium red plant and a white plant.

Answer 1

The ratio of phenotypes in the F2 generation indicates that two genes are involved in the production of color in wheat, with a total of four additive alleles influencing the phenotype. True-breeding medium red (RaRa) crossed with white (rraa) would yield medium-red F1 offspring. F2 would show a ratio of 3 medium-red to 1 white.

Step-by-step explanation:

The phenotypic ratio of the F2 generation, which is given as 1 dark-red : 4 medium-dark-red : 6 medium-red : 4 light-red : 1 white, suggests the involvement of two genes in color production due to the pattern not matching a simple 3:1 monohybrid cross. It fits a modified dihybrid cross ratio of 1:4:6:4:1, which is reminiscent of a 9:3:3:1 ratio with intermediate forms due to incomplete dominance.

To determine the number of additive alleles needed for each phenotype:

Dark-red: All four additive alleles are present (2 from each gene).

Medium-dark-red: Three additive alleles are present.

Medium-red: Two additive alleles are present.

Light-red: One additive allele is present.

White: No additive alleles are present.

We can assign the symbols R for the allele contributing to red color and r for its absence from one gene, and A for the allele contributing to red color and a for its absence from the second gene.

Thus, possible genotypes for medium red are RaRa or rrAA. For light red, the genotype could be Ra with the other gene being rr or aa, such as Rraa or Aarr.

For a cross between a true-breeding medium red plant (RaRa) and a white plant (rraa), the F1 would all be Rara (medium-red), and the F2 generation would show a 3:1 phenotypic ratio of medium-red (R-) to white (-rr) since the white parental plant can only contribute recessive alleles.

Answer 2

a). 2 genes are involved in production of colour in F2 generation.

b). 2 possible alleles for each phenotype

c).Rr (medium red), rr light red

d) F1 generation 1:2:1 ( 1 light red stain, 2 medium red) and F2 generation 1:1

1 medium red and 1 light red.

Step-by-step explanation:

F1 generation:

dark red strain X white strain

RR X rr

r r

R rR Rr

R rR Rr

genotype ratio 4:1, all medium red

Cross between F1 generation of true breeding plants

R r

R RR Rr

r Rr rr 1 light red stain, 2 medium red, 1 white genotype ratio 1:2:1

In F2 Generation, F1 plants interbred ie true breeding

F2 generation:

R r

r Rr rr

r Rr rr

It produces 1 medium red and 1 light red plant. genotype ratio 1:1

Here alleles Rr is for medium red and rr is for light red

Additive allele depends on the allele concentration for a phenotype trait to appear.