Question

2. Labrador retriever dogs have three coat colors: black, brown and yellow. The color of the pigment deposited in the hairs is determined by a gene with two alleles (B or b). The B allele codes for black pigment and is dominant to the b allele which codes for brown. Another gene with two alleles (E or e) determines if the pigment created will be deposited into the hairs. If both copies (ee) are recessive, no pigment will be deposited (regardless of which is produced), and the dog will be yellow.

1. What do you call the type of relationship that the E gene has with the B gene? (0.5 points)

2. You cross together two Labradors with genotypes B/b; E/e.
   1. What is the phenotype of the B/b; E/e genotype? (1 point)
   2. Determine the phenotypic ratio for the F1 generation. Show your Punnett square! (2 points)

3. What phenotypic ratios do you get if you cross the following types of Labradors (show your Punnett square as well; 6 points):
   1. B/b; E/e X b/b; e/e
   2. B/B; E/E X B/B; E/e
   3. b/b; E/E X B/B; e/e

Answer 1

a) epistasis, E is the epistatic locus when the genotype at the E locus is ee, then phenotype is yellow regardless of the genotype at the the B locus, so this is an example of recessive epistasis.

b) i) B/b E/e has dominant alleles at both loci, so phenotype is black.

ii) B/b E/e * B/b E/e

	BE	Be	bE	be
BE	BBEE (black)	BBEe (black)	BbEE (black)	BbEe (black)
Be	BBEe (black)	BBee (yellow)	BbEe (black)	Bbee (yellow)
bE	BbEE (black)	BbEe (black)	bbEE (brown)	bbEe (brown)
be	BbEe (black)	Bbee (yellow)	bbEe (brown)	bbee (yellow)

Black:Yellow: brown=9:4:3

c) i) B/b; E/e X b/b; e/e

	BE	Be	bE	be
be	BbEe (black)	Bbee (yellow)	bbEe (brown)	bbee (yellow)

phenotypic ratio is

Black:Yellow: brown=1:2:1

ii) B/B; E/E X B/B; E/e

	BE	BE
BE	BBEE (black)	BBEE (black)
Be	BBEe (black)	BBEe (black)

phenotypic ratio

Black:Yellow: brown=1:0:0

iii) b/b; E/E X B/B; e/e

	bE	bE
Be	BbEe (Black)	BbEe (Black)
Be	BbEe (Black)	BbEe (Black)

the

phenotypic ratio

Black:Yellow: brown=1:0:0