Page 66 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

58 Chapter 3 Extensions to Mendel’s Laws

Figure 3.12 Recessive epistasis determines coat color in Labrador retrievers. (a) Labrador retriever colors. (b) Yellow Labrador
retrievers are homozygous for the recessive e allele, which masks the effects of the B or b alleles of a second coat color gene. In E– dogs, a
B– genotype produces black and a bb genotype produces brown.
a: © Vanessa Grossemy/Alamy
(a) Chocolate, yellow, and black Labrador retrievers (b) A dihybrid cross showing recessive epistasis

P BB EE bb ee

Gametes B E b e

F 1 (all identical)
Bb Ee Bb Ee

F 2
B E B e b E b e

B E BB EE BB Ee Bb EE Bb Ee
9 B– E– (black)
3 bb E– (chocolate) B e BB Ee BB ee Bb Ee Bb ee
3 B– ee (yellow)
1 bb ee b E Bb EE Bb Ee bb EE bb Ee

b e Bb Ee Bb ee bb Ee bb ee

Recessive epistasis these F 1 dihybrids produce an F 2 generation with nine black
We present here three examples of recessive epistasis, where dogs (B– E–) for every three brown (bb E–) and four yellow
homozygosity for a recessive allele of one gene hides the ef- (– – ee) (Fig. 3.12b). Note that only three phenotypic classes
fect of a second gene. In other words, when an individual is exist because the two genotypic classes without a dominant E
homozygous for the epistatic recessive allele of the first gene, allele—the three B– ee and the one bb ee—combine to pro-
the phenotype is independent of the alleles present at the sec- duce a yellow phenotype. The telltale ratio of recessive epis-
ond (hypostatic) gene. The final example in this section de- tasis in the F 2 generation is thus 9:3:4, with the 4 representing
scribes a surprising phenomenon in which recessive epistasis a combination of 3 (B– ee) + 1 (bb ee). Because the ee geno-
is reciprocal between the two genes that determine the trait. type masks the influence of the other gene for coat color, you
cannot tell by looking at a yellow Labrador whether its geno-
Yellow Labrador retrievers The sleek, short-haired coat of type at the B locus is B– (black) or bb (chocolate).
Labrador retrievers can be black, chocolate brown, or yel- Scientists understand with some precision the bio-
low (Fig. 3.12a). Which color appears depends on the al- chemical pathways in which different alleles of the B and E
lelic combinations of two independently assorting coat genes operate (Fig. 3.13). All coat color in dogs comes
color genes (Fig. 3.12b). When the dominant E allele of the from two pigments synthesized from a common precursor:
first gene is present, the B allele of the second gene deter- a dark pigment called eumelanin and a light pigment called
mines black, and the recessive bb homozygote is chocolate. pheomelanin. When Labrador retrievers have at least one
However, a double dose of the recessive allele (ee) hides copy of the E allele, the resultant protein E ensures that the
the effect of any combination of the black or chocolate al- animals will make only eumelanin and no pheomelanin.
leles to yield yellow. Thus, the recessive ee homozygous The protein specified by the B allele is required for
genotype is epistatic to any allelic combination at the sec- eumelanin synthesis and its deposition in the hair, while
ond, hypostatic gene, B. the protein made by the b allele is less efficient. As a result,
Let’s look at the phenomenon in greater detail. Crosses chocolate E– bb dogs have less eumelanin in their hairs
between pure-breeding black retrievers (BB EE) and one type than black dogs with at least one B allele (E– B–). But in
of pure-breeding yellow retriever (bb ee) create an F 1 genera- the absence of the E protein (in ee dogs), only pheomelanin
tion of dihybrid black retrievers (Bb Ee). Crosses between is synthesized, and so the dogs appear yellow. It is easy to