Coat Color Genetics

Question 1

What percentage of the mouse coat are overhairs and underhairs? What role does each play in coat color?

Answer 1

Overhairs = 20%. Major role in coat color
Underhairs = 80%. Minor role in coat color

Question 2

Describe the structure of the different types of overhairs (3) and underhairs (1).

Answer 2

Overhairs:
Monotrich - Guard hairs
Awls - No constriction
Auchenes - Single constriction

Underhairs:
Zigzags - Shorter, with 3 flat constrictions

Question 3

What are the two types of pigment?

Answer 3

Eumelanin - black
Phaeomelanin - Yellow

Question 4

What is epistasis in relation to coat color genetics?

Answer 4

Alleles at one locus can obscure or change expression of alleles at other loci.

Question 5

What are the five basic colors and on which chromosomes are they located?

Answer 5

Agouti (A) - A locus, chromosome 2A
Brown (B) - B locus, chromosome 4
Albino (C) - C locus, chromosome 7
Eye color locus (P) - Chromosome 7
Blue (D) - D locus, chromosome 9

Question 6

What pigments does the agouti locus control? What is the dominant and recessive colors?

Answer 6

Amount and distribution of phaeomelanin and eumelanin.
A - Agouti. Banding of hair strands
a - Nonagouti. Removes banding. Recessive.

Question 7

What allele mutations exist for the agouti locus?

Answer 7

A(w) - White bellied agouti. Dominant to A(+)
A(+) - Yellow bellied agouti. Recessive to A(w)

A(y) - Yellow agouti, aka lethal yellow. Homozygotes embryonic lethal. Dominant to everything.
A(vy) - Viable yellow. Associated with obesity.

a(t) - Black and tan. Recessive to A on dorsum. Dominant to A on ventrum

a(e) - Extreme non-agouti. Jet black. Eliminates any yellow strands.
a(x) - Lethal nonagouti. Jet black. Homozygotes embryonic lethal.

Question 8

What is the B locus also known as?

Answer 8

Brown locus, tyrosinase-related protein 1, Tyrp1

Question 9

What is the tyrosinase-related protein gene responsible for?

Answer 9

Catalyzes oxidation of dihydroxyindole carboxylic acid. Determines production of eumelanin.

Question 10

What are the main alleles at the B locus?

Answer 10

Tyrp1(B) - Black. Dominant
Tyrp1(b) - Brown. Recessive. Wild mouse color. Dilutes black to brown.

Question 11

What allele mutations exist for the B locus?

Answer 11

Tyrp1(B-w) - White based brown, semi-dominant. Causes pigment reduction at hair base
Tyrp1(B-lt) - Light
Tyrp1(b-c) - Cordovan. Reddish-brown

Question 12

What is the C locus also known as?

Answer 12

Albino locus, tyrosinase, Tyr

Question 13

What is the tyrosinase gene responsible for?

Answer 13

Catalyzes conversion of tyrosine to DOPA (dihydroxyphenylalanine) and DOPA to DOPA-quinone. Permits or prevents color expression.

Question 14

What is the structural underpinning of albinism?

Answer 14

Not a lack of melanocytes, but from an alteration of structure of tyrosinase in otherwise normal melanocytes.

Question 15

What are the main alleles at the C locus?

Answer 15

Tyr (C) or (+) - Color, dominant
Tyr (c) - Albino, recessive. If homo, overrides all other coat color genes. Poor performance on intelligence tests due to visual impairments.

Question 16

What allele mutations are found at the C locus?

Answer 16

All are recessive.
Tyr(c-ch) - Chinchilla. Reduces agouti and black to silver.
Tyr(c-e) - Extreme dilution (light gray w/ black eyes)
Tyrc-h - Himalayan. White until first molt then dark nose, ears, and feet.

Question 17

What is the D locus also known as?

Answer 17

Dilute locus, Myo5a.

Question 18

What is the function of the Myo5a gene?

Answer 18

Non-filament forming unconventional myosin protein. Controls intensity of pigment specified by other genes.

Question 19

What are the major alleles at the D locus?

Answer 19

Myo5a(+) - No dilution. Dominant
Myo5a(d) - Dilution. Recessive. In non-agouti mice, causes hairs to be blue or blue-black. Dilutes yellow to buff. Linked to short ear gene.

Question 20

What allele of the D locus is linked to a condition in humans?

Answer 20

Myo5a(d) - Dilution. Model of Griscelli syndrome, where infants have hypopigmented skin and silver-gray hair in infancy.

Question 21

The D locus only appears on what background?

Answer 21

Tyr(+/+) mice - Those with color

Question 22

What is the P locus? What gene is responsible, and what is its function?

Answer 22

Pink eye locus. Oca2 gene, a melanosomal transmembrane protein.

Question 23

What are the major alleles at the Oca2 locus?

Answer 23

Oca2(P) - Normal colored eyes. Dominant.
Oca2(p) - Pink-eyed dilution. Recessive. Also reduces pigmentation of coat - more dramatically in eumelanin than phaeomelanin.

Question 24

Is an OCA2(p) eyed-mouse the same as an albino?

Answer 24

No. p/p still has small amounts of melanin in the iris and retina.

Question 25

What are allele mutations at the P locus?

Answer 25

p(d) - Dark pink eye dilution.

Question 26

What is the Ednrb locus? Where is it found? What is the gene responsible for?

Answer 26

Piebald locus. Enrb gene is responsible for endothelial receptor type B, which controls distribution of coat pigment.

Question 27

What are the major alleles at the Ednrb locus?

Answer 27

Enrb(+) - No piebald spotting. Dominant.
Enrb(s) - Piebald spotting. Recessive. Distinct white spots with sharp borders. 10% of homos may develop aganglionic megacolon.
Enrb(s-l) - Piebald lethal. Recessive. White with occasional patches of pigmented hairs (usually on head or rump). 100% of homozygotes with eventually succumb to aganglionic megacolon - Model for Hirschsprung disease.

Question 28

Describe agouti coloration.

Answer 28

Alternating patterns of yellow distal portion of the hair and black proximal portions.

Question 29

Describe the interaction of agouti protein with alpha-melanocyte stimulating hormone.

Answer 29

Agouti protein A binds alpha-mSH and activates tyrosinase. Activation required to make pheomelanin and eumelanin. Myo5a protein functions to transport pigment to keratinocytes.