Question

A particular flower can be purple, blue, red, or white. Two different pure-breeding white plants are crossed and the F1 are then crossed to produce an F2 generation. What might a 9:7 phenotypic ratio in the F2 indicate.

- additivity
- dominant epistasis
- reciprocal recessive epistasis
- co-dominance
- recessive lethality

Answer 1

A 9:7 phenotypic ratio in the F2 generation indicates reciprocal recessive epistasis, where two genes interact such that their recessive alleles can suppress a certain phenotype when both are homozygous. This is different from simpler Mendelian ratios and hints at a more complex genetic interaction.

Step-by-step explanation:

If we observe a 9:7 phenotypic ratio in the F2 generation from crossing two different pure-breeding white plants, it suggests a specific pattern of genetic interaction. This ratio does not match the simple 9:3:3:1 ratio expected from a Mendelian dihybrid cross with independent assortment of two genes, each with dominant and recessive alleles. Instead, a 9:7 ratio is consistent with reciprocal recessive epistasis, where two genes interact such that the recessive alleles of both genes are required to produce a certain phenotype (in this case, the white flowers).

An example can be imagined where one gene, when homozygous recessive, could block the expression of a second gene. If both genes must be active for the development of the non-white colors, but each has recessive alleles that alone can revoke color when homozygous, the result would be a significant number of white flowers.

Therefore, reciprocal recessive epistasis is the best fit for explaining this 9:7 phenotypic ratio in the F2 generation of this flower's cross.