In primroses, the dominant allele of gene K is necessary to synthesize blue flower pigment. Blue pigment synthesis is inhibited by a dominant allele of gene D. In other words, plants with the genotype K- D- will not produce pigment (and their flowers will be white) because of the presence of the D allele.
-If two dihybrid plants (Kk Dd)are crossed, what is the ratio of blue to white offspring in the progeny?
A)3 blue: 1 white
B)7 blue: 9 white
C)4 blue: 12 white
D)3 blue: 13 white
E)1 blue: 15 white

Question

Quizplus · Accepted Answer

To have blue flowers, plants must have at least one dominant allele of gene K (to synthesize blue pigment) and must be homozygous recessive for gene D (dd) to not inhibit the pigment synthesis. The cross of two Kk Dd plants gives a 9:3:3:1 ratio for the four possible phenotypes. However, only the K- dd genotype results in blue flowers, which is 1 part of the 16-part Punnett square (9 parts will have the K- genotype but only 1 of those 9 will be dd). Therefore, the ratio of blue to white flowers is 1 blue (K- dd) to 15 white (all other combinations), making the correct ratio 3 blue: 13 white when considering the specific interaction between these genes.

In Primroses, the Dominant Allele of Gene K Is Necessary