Solutions For An Introduction to Genetic Analysis Chapter 6 Problem 41P

While cross 4 gives a new situation. A difference in the color of F₁ from that of either of the parent shows that there are two separate genes involved, which might be either codominant or incompletely dominant. When such a phenomenon occurs, the normal 9:3:3:1 ratio will not be obtained in F₂. In cross 4, the progeny are in the ration of 9:4:3, showing that there might be epistasis. Also for the three types phenotypes present in the F₂ generations shows that the two parents in the F₁ generation should be heterozygous.

Let ‘a’ stand for the scarlet gene, while A for its colorless allele. Assume that there is a dominant allele C, which blocks the allele of our interest.

Cross 4

Parent generation c^o/c^o; A/A X C/C; a/a

F₁ generation C/c^o; A/a

F₂ generation

9 C/- ; A/- yellow

3 C/- ; a/a scarlet

3 c ^o /c ^o ; A/- orange

1 c ^o /c ^o ; a/a orange (epistasis – c^o blocking the expression of a/a)

So, the result shows that 9 C/- ; A/-, 3 C/- ; a/a, 3 c^o/c^o ; A/-, 1 c^o/c^o ; a/a