Coat Color Genetics Flashcards
melanocytes
pigment producing cells in mammals
what are melanocytes derived from
melanoblasts (neural crest cells)
how do genes affect melanocyte distribution in the body
controls the number, migration, and survival of melanocytes
where is the first area that white spots will appear
periphery
- farthest distance from the neural crest
what are pigments derived from
tyrosine
how do pigments get distributed to hair
pigments get deposited in granules (melanosomes) that get distributed along the hair shaft
what are the two main pigments
pheomelanin
eumelanin
pheomelanin
yellow, red, white pigments
eumelanin
black pigment
what causes banding pattern of hair
timing of pigment distribution during hair growth
banding:
black (eu) –> yellow (pheo) –> black (eu)
what are the two main genetic controllers of pigment
MC1R
ASIP
MC1R
melanocortin receptor-1
G protein coupled receptor
- MSH binds MC1R
- MC1R triggers increase in cAMP
- signals eumelanin production
ASIP
agouti signaling protein
- ASIP binds to MC1R and blocks MSH binding
- decreases cAMP
- signals pheomelanin production
what are the two ASIP transcription promoters
ventral promoter
hair cycle promoter
ventral promoter
if transcription of ASIP begins at the ventral promoter –> pheomelanin will expressed on ventrum
medium expression: pheomelanin on ventral muzzle, distal limbs, and chest
high expression: pheomelanin everywhere except dorsum
hair cycle promoter
if transcription of ASIP begins at the HCP –> pheomelanin will be expressed in a banding pattern across the body
medium expression: banded, dark
high expression: banded, light
what are the two ways of getting yellow coat color
- MC1R loss of function
- ASIP gain of function
MC1R loss of function
yellow
common but recessive
- decreased expression of MC1R
- no cAMP increase
- pheomelanin
ASIP gain of function
yellow
less common but dominant
(requires increased expression from both promoters)
- overexpession of ASIP over entire body
- blocks MSH at all MC1R
- pheomelanin
what are the 3 ways of getting black coat color
- ASIP loss of function
- MC1R gain of function
- K locus
ASIP loss of function
black
more common but recessive
- decreased expression of ASIP
- no MC1R blockage
- eumelanin
MC1R gain of function
black
least common
- MC1R blocks ASIP binding
- no MSH block
- eumelanin
K locus
black
MOST common in dogs - dominant
beta defensin mutation (CBD103) prevents ASIP from binding to MCR1
- independent of MC1R and ASIP status
how does MC1R status compare to ASIP and K locus status
epistatic
if no MC1R present –> will always get PHEOMELANIN because no cAMP signaling
what causes albinism
tyrosinase mutation leading to absent pigment production
recessive (rare)
pleiotropic - affects eyesight
OCA type 1
oculocutaneous albinism
caused by tyrosinase mutations
ex. temperature sensitive mutations in siamese cats (white + diluted black areas in cold regions)
what causes brown coat color
TRP-1 mutation leading to inability to catalyze final step of eumelanin production –> “less intense” black pigment
no pleiotropic effects
what causes cream coat color
OCA type 4
mutations at MATP or SLC45A2 that disrupts tyrosinase trafficking and proton transport to melanosomes
what causes dilute coat color
melanophillin mutation that disrupts melanosome transport in melanocytes –> causes clumping of pigment granules leading to dilute color
recessive
bad pleiotropic effects
- lavender foals –> neurologic
- collies –> cyclic neutropenia
pleiotropy
the production of two or more apparently unrelated effects by a single gene
how does white spotting occur
mutations that cause failure of melanocytes to reach certain areas/decreased survival
dominant white spotting
tobiano in horses
recessive white spotting
white spots in cats, dogs
pleiotropic effects of white spotting
deafness
- inner ear (stria vascularis) requires melanocytes
- no melanocytes migrating to ear (white ears) –> prone to deafness
lethal white foal syndrome
overo x overo –> offspring:
- solid
- overo
- overo
- lethal white
foals have ganglionic megacolon leading to colic immediately after birth (inappropriately innervated gut)
concurrent deafness
