Question

1. Labrador retrievers may be black, brown or golden. While each color may breed true, many different outcomes occur if many different litters are examined from a variety of matings, where parents aren’t necessarily true breeding. Below are some of the multiple possibilities. Propose a mode of inheritance and determine the genotypes of the parents in the crosses. (4)

a) Black             x          brown              =          all black

b) Brown          x           Black                =          ½ black

                                                                        ½ brown

c) Black             x          Brown              =          ¾ black

                                                                        ¼ golden

d) Black            x          Golden             =          all black

Answer 1

If you notice in the cross “a”, we obtained all black. This means that both parents are true breeding and the dominant color is the expressed. We can make the Punnett Square of each cross to test the inheritance pattern. However, if we see more than two parents of phenotype, the most probable explanation is that the color pattern is governed by more than two alleles, or multiple alleles. In this context, lets try with epistasis, where a gene is governed by the expression of a second gene.

B= Black

b=Brown

C=Dominant non epistatic allele (do not influence the color pattern)

c=Recessive allele that will interfere with the expression of black and brown color, leading to a golden color

In conclusion, the inheritance of color in Labrador is governed by two genes. One gene that codes for brown and black color. A second gene that regulates the expression of of brown and black color (Epistatic gene). If the epistatic gene has a homozygous recessive genotype, it will mask the expression of brown and black color, leading to golden color.