IMAT 2015 Q25 [Phenotypes]

The genotype QqRr produces a certain phenotype. If two individuals with a genotype of QqRr reproduce, how many different possible phenotypes could be created, assuming all allele combinations are equally viable and the phenotypes are the result of complete dominance?

A. 16
B. 4
C. 5
D. 8
E. 9

The answer to this question is choice B, 4 phenotypes.

This question might seem confusing at first, but it would be useful to figure out what the question is asking for:

  1. "Genotype QqRr" : the fact that there are 2 genes with 2 alleles each, should prompt you to think that this is indeed a dihybrid cross.
  2. "…how many different possible phenotypes" : Now that you know that this is a dihybrid cross, you would now need to discern the different genotypes. But you don’t stop at genotypes, what’s being asked of you is phenotypes, which means that you will need to discern the different phenotypes that will come out of the different genotypes.

Before we rationalize the answer, there is a bit of a shortcut, but it requires you to memorize this characteristic fact of dihybrid crosses.

  • As you know this is a dihybrid cross. Characteristically, if there are 2 heterozygous parents, QqRr in the case of this question, their cross will yield the phenotypic ratio of 9:3:3:1. That’s 4 numbers, which makes sense that choice B is the answer, since there are 4 different phenotypes included in that phenotypic ratio.

Let us then back that up with a punnett square:

  • At the left side of the image above, you will see the plotted out punnett square based on the genotype of the parents. If you list out the different unique genotypes that each allelic combination can give you, it will be listed out like the table seen in the middle of the image. However remember, what’s being asked here is phenotype, which means that you are only concerned about which alleles are expressed within a given genotype. As you know, dominant alleles are expressed regardless if heterozygous or homozygous, while recessive alleles are only expressed if they are a homozygous pair. If you observe the table at the middle of the image, and analyzed the different genotypes that expressed the same type of the 2 traits, you will come up with the phenotypic ratio of 9:3:3:1, as supported by the table on the right side of the image.