Question

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?

- 4 blue: 12 white
- 1 blue: 15 white
- 3 blue: 1 white
- 3 blue: 13 white
- 7 blue: 9 white

Answer 1

Using a Punnett square for the dihybrid cross, the ratio of blue to white primrose offspring is 3 blue: 13 white.

Step-by-step explanation:

The student's question pertains to a dihybrid cross between two primroses with genotypes Kk Dd. Since the dominant allele of gene K is required for blue pigment synthesis and the dominant allele of gene D inhibits this synthesis, the plants will have blue or white flowers depending on these alleles. To solve this, we can create a Punnett square with all possible gametes from the parents.

Each parent can produce four types of gametes: KD, Kd, kD, kd. Crossing these results in the following genotypes for the offspring:

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The pattern continues with only kk genotype plants expressing blue flowers, as any genotype with a D results in white flowers. After completing the Punnett square, we find that three genotypic combinations yield blue flowers: Kkdd, kkDd, and kkdd. Combining the probabilities, we see:

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Adding them up, the total probability of a blue flower offspring is 3/16. Conversely, the probability of white flowers (everything else) is 13/16.

Therefore, the ratio of blue to white offspring in the progeny from the given dihybrid cross is 3 blue to 13 white, which is the correct option among the choices provided.

The correct ratio of blue to white offspring is 3 blue: 13 white.