Question

In Labrador retrievers, there are 3 gene products involved in determining coat color:  gene product B, gene product E, and gene product C.  B produces a melanin pigment such that B/B and B/b will produce a black lab while b/b (with less melanin produced) will result in a chocolate lab.  However, for the B gene to be expressed, a signaling hormone (gene E product) must be produced to activate transcription of the B gene.  E is fully dominant to e (loss of function mutation).  In the absence of the hormone, a yellow lab will result (due to the pigment product of the C gene).  What are the ratios of black, chocolate, and yellow labs that will be produced from a BbCCEe x BbCCEe cross?  Hint:  set up a Punnett square and determine the phenotypes of each of the 16 boxes.   What type of genetic phenomenon is occurring here with respect to the E gene?  Explain why you think this is.

Answer 1

In this question, I've first remove CC from the genotype because both the parents are homozygous recessive for CC which means that every offspring will be CC genotype only. This reduces the number of punett square as with 3 genes, punett square have 64 boxes and even in the question, it is asked for 16 boxes which comes from a dihybrid cross. So don't think it's wrong or there's a mistake. Basically we can safely assume everyone will have CC gene product.

In the above image, I've performed punett square for the parental genotypes.

I'll attach another image below with every box written on top what will be the phenotype for better understanding.

In this image I've written down the phenotype over the genotype. For example, the first offspring is BBEE and the phenotype written over it is Black.

Now as for the ratio of phenotype is :-

9 Black : 3 Chocolate : 4 Yellow

Question b) Type of genetic phenomenon occuring here with Respect to E.

The answer is Epistasis. Epistasis is occuring here because in Epistasis expression of one gene is dependent on other genes' expression and if other genes are not expressed they will affect the phenotype of the genes in questions. In our question, even though BB alleles are present, Labrador is Yellow because ee is homozygous recessive and even though Black pigment is produced by BB gene, mutation in E gene is affecting the Black colour expression.