Question

1. Genes often interact with one another. The term epistasis is applied to cases in which one gene alters the expression of another gene that is independently inherited. In Labrador retriever dogs one gene locus is involved with production of melanin pigment: B (black) and b (brown) are its two alleles. Another gene locus determines whether the melanin produced is actually deposited in individual hairs as they grow (E) or not deposited (e). Any dog with at least one dominant allele B and one dominant allele E will be a black Labrador. A dog with the homozygous recessive bb and at least one dominant E will be a lighter, chocolate Labrador. If a dog has the homozygous ee genotype it will be a yellow Labrador, regardless of the alleles at the pigment locus.

   1. Give possible genotypes for a dog with the black Labrador phenotype?

      With respect to locus B

      With respect to locus E

   2. Give possible genotypes for a dog with the chocolate Labrador phenotype?

      With respect to locus B

      With respect to locus E

   3. Give possible genotypes for a dog with the yellow Labrador phenotype?

      With respect to locus B

      With respect to locus E

2. A yellow Labrador with the genotype bbee mates with a black Labrador that is homozygous for both alleles. What are the ratios of genotypes and phenotypes expected in the F1 and F2 generations?

P.S More in Need for help with Question 2.

Answer 1

1.Give possible genotypes for a dog with the black Labrador phenotype?

In the question it is given that for black phenotype, there has to be at least one dominant B allele and one dominant E alleles. So the genotypes will be as follows :

With respect to locus B - BB / Bb

With respect to locus E - EE / Ee

2.Give possible genotypes for a dog with the chocolate Labrador phenotype?

In the question it is given that for chocolate phenotype , there has to both recessive b allele and at least one dominant E allele. So the genotypes will be as follows :

With respect to locus B - bb

With respect to locus E - Ee / ee

3.Give possible genotypes for a dog with the yellow Labrador phenotype?

Since in the question it is mentioned that both e recessive alleles have to be present and the genotype of B allele doesn't matter, the genotypes will be as follows :

With respect to locus B - BB / Bb / bb

With respect to locus E - ee

From the above observations , it can be concluded that the coat colour in Labradors follows an inheritance pattern of Recessive epistasis. Recessive epistasis can be defined as an inheritance pattern where both the non-allelic genes involved have to be present in dominant form for the correct phenotype expression. Here the recessive allele of one gene masks the effect of the allele of other gene. Thus one phenotype can express itself independently of the other while the other phenotype cannot do the same.

4. A yellow Labrador with the genotype bbee mates with a black Labrador that is homozygous for both alleles. What are the ratios of genotypes and phenotypes expected in the F1 and F2 generations?

Please see the image below for explanation :