Page 84 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

76 Chapter 3 Extensions to Mendel’s Laws

Figure 3.30 Dog color pattern is a polygenic trait. The because eumelanin is densely deposited, and their E– bb
major alleles determining three kinds of dog coat color patterns. counterparts are brown because less eumelanin is produced
(top): © Tierfotoagentur/Alamy; (middle): © Vanessa Grossemy/Alamy; and it is deposited less densely. Because the role of TYRP1
(bottom): © Martin Rogers/Getty Images
depends on the presence of MC1R (made by gene E), ee is
epistatic to both B and b (recall Figs. 3.12 and 3.13).
Gene D specifies Melanophilin (MLPH), another pro-
tein required for pigment deposition. The recessive allele
of gene D specifies a version of MLPH that functions less
efficiently than the MLPH specified by the dominant al-
lele. The lower amount of MLPH activity in dd homozy-
gotes results in less pigment deposition and thus dilution
of the colors dictated by the other genes. The dominant
(normal) allele (D) does not dilute the colors. For exam-
ple, an E– B– D– dog is black, while an E– B– dd dog is
light black.
m
Mask (E -)
Genes S and M control spotting
Dogs homozygous for the recessive allele of gene S (that is,
p p
s s ) are white with large spots of color; this pattern is
called piebald (Fig. 3.30). As long as a dog has one domi-
nant S allele, it will not be piebald. Gene S specifies a pro-
tein called MITF, a transcription factor needed to express
(transcribe) many genes specifying enzymes needed for
pigment production. The s allele makes a version of MITF
p
that is less active than normal. Melanocyte precursor cells
with only low levels of MITF die, resulting in white areas
p p
Piebald (s s ) of the skin with no melanocytes and thus no pigment. By
chance, some melanocyte precursor cells have sufficient
MITF to survive, producing colored spots; the color is
determined by genes other than S.
A second gene called M also controls the patterning of
2
pigmentation, and it has codominant alleles M and M .
1
The M allele is normal—it specifies a protein called
2
PMEL required for eumelanin deposition. The M allele
1
makes an abnormal PMEL protein that interferes with eu-
1
2
melanin deposition and thus dilutes color. M M heterozy-
gotes, called merle dogs, have patches of diluted color (the
1
M phenotype) and patches of normal color (the M pheno-
2
2
1
Merle (M M ) type) (Fig. 3.30). Breeders would never mate two merle
1
dogs because the M gene is pleiotropic. The M allele has
recessive deleterious effects: So-called double merle dogs
1
(M M ) usually have serious health problems, including de-
1
allows ASIP to inhibit MC1R sometimes, permitting phe- fects in hearing and vision. The eye and ear problems are
y y
omelanin production. As a result, k k homozygotes allow due to the death of retinal and ear pigment cells caused by
expression of the fawn, agouti, or tan traits associated with the abnormal PMEL protein.
t
w
Y
the A , a , or a alleles of the A gene. This example of coat color in dogs gives some idea of
the potential for variation from just half of the genes known
to affect coat color. Amazingly, this is just the tip of the
Genes B and D control deposition of all pigments iceberg. When you realize that both dogs and humans carry
Gene B specifies TYRP1, a multifunctional protein re- roughly 27,000 genes, the number of interactions that con-
quired for eumelanin synthesis and for depositing pigment nect the various alleles of these genes in the expression of
in melanocytes. The B allele makes fully functional phenotype is in the millions, if not the billions. The potential
TYRP1, and the b allele specifies a less active version. As for variation and diversity among individuals is staggering
described earlier, E– B– Labrador retrievers are black indeed.