Page 82 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

74 Chapter 3 Extensions to Mendel’s Laws

TABLE 3.3 Some of the Genes Affecting Domestic Dog Coat Color and Pattern
Dominance
Gene Protein Series of Alleles Phenotypes
Pigment-type switch genes
t
w
y
y
Gene A Agouti Agouti signaling A > a > a > a A Fawn (lots of light pigment on hair)
w
protein (ASIP) a Agouti (light stripe on dark hair)
a t Tan belly (only hairs on belly have some
light pigment)
a Black or brown (no light stripe on hairs)
m
m
Gene E Extension Melanocortin E > E > e E Black mask on fawn or brindle
receptor (MC1R) E Eumelanin (dark) and pheomelanin
(yellow) pigments
e Only pheomelanin (cream, tan, red)
y
br
b
b
Gene K Kurokami Beta-defensin K > k > k K Solid color
k br Brindled
k y Gene A markings expressed normally
Dilution genes
Gene D Dilute Melanophilin (MLPH) D > d D Colors not dilute
d Colors dilute
Gene B Brown Tyrosine-related B > b B Black: eumelanin deposited densely
protein (TYRP1) b Brown: eumelanin deposited less densely
Pigment cell development
and survival genes
p
Gene S Spotting Micropthalmia-associated S > s S No white markings
p
transcription factor (MITF) s Colored patches on white background
1
2
1
Gene M Merle Premelanosome M = M M Coat color diluted (homozygote
protein (PMEL) has various health problems)
2
M Normal color
Genes A, E, and K control the switch from a allele of gene A, which specifies an inactive ASIP pro-
eumelanin to pheomelanin production tein, and for the K allele of gene K, whose product is a
b
Skin cells called melanocytes make the pigments deposited functional Beta-defensin. Labradors that are E– produce
in each dog hair. Melanocytes can produce either a dark eumelanin because MC1R is present and is switched on by
b
pigment (eumelanin), or a light pigment (pheomelanin). the K Beta-defensin protein (Fig. 3.29b). These E– dogs
The MC1R protein, specified by the E gene, spans the cell are either black or chocolate depending on their gene B
membrane and acts a switch that determines which pig- allele (as will be explained). In contrast, the melanocytes of
ment a melanocyte produces (Fig. 3.29a). Melanocytes ee Labradors have no MC1R, so the switch cannot be
produce eumelanin only when MC1R is switched on; phe- turned on; pheomelanin is made by default, and the dogs
omelanin is produced when the switch is turned off. By are yellow. (The particular shade of yellow in ee dogs,
binding to MC1R at the cell surface, two proteins made by which varies from cream to red, is controlled by other
nearby skin cells control the MC1R switch. Binding of genes not yet identified at the molecular level.)
ASIP (specified by gene A) turns the switch off, but when Other dog breeds can have various alleles of genes A,
Beta-defensin (specified by gene K) successfully out- E and K. Four different alleles of the A gene form a domi-
competes ASIP for MC1R binding, the switch flips on. The nance series (Table 3.3). As just described, the a allele
different alleles of the E, A, and K genes found in different makes a nonfunctional protein. The proteins made by the
dog breeds result in various colors and pigment distribu- other three alleles direct the pigment switch with different
tion patterns on each hair and over the dog’s body as a efficiencies or in different parts of the dog’s body. Although
whole. the A gene in dogs is the same as the A gene in mice (review
As described earlier, Labrador retrievers have two dif- Fig. 3.7), the A allele behaves differently. In contrast with
Y
Y Y
ferent gene E alleles; E specifies MC1R while e is nonfunc- mice, dogs may be homozygous A A ; they are an overall
tional. All Labrador retrievers are homozygous for the light brown color called fawn because the hairs contain a