Question

You identify a rare purple plumed parrot. When you cross the parrot to a true breeding white parrot, you get ½ white and ½ purple parrots. When you cross two purple parrots you get 1/3 white parrots and 2/3 purple parrots. Which of the following best explains this result?a. The purple allele is lethal in the homozygous state.b. White is dominant to purple.c. The white allele is lethal in the homozygous state.d. The purple allele is lethal in the heterozygous state.

Answer 1

Answer:

a. The purple allele is lethal in the homozygous state

Explanation:

You cross a purple parrot with a true breeding (homozygous) white parrot and you get 1/2 purple parrots and 1/2 white parrots. This result can only happen if the purple parrot is heterozygous and the white parrot is homozygous recessive (if any of the parents had been homozygous dominant, 100% of the offspring would have had the dominant phenotype).

Then: Pp (purple) X pp (white) ---> 1/2 Pp, 1/2 pp

When you cross two purple parrots you get 1/3 white parrots and 2/3 purple parrots. The 2:1 phenotypic ratio is typical of a lethal allele in the homozygous state.

In a cross of two purple parrots: Pp

X Pp, one would expect the genotypic ratios 1 PP : 2 Pp : 1pp and the phenotypic ratios 3 Purple : 1 white.

But the PP genotype is lethal, so those individuals die and the only purple parrots we see are the heterozygous ones.