Coat Color Genetics Flashcards
what kind of trait is coat color and why?
qualitative; even though there are so many genes involved it still can be put into categories
what are the 2 starting point options for coat color for horses that all other genes only modify?
black or red
how are genes named? (2)
- in reference to their role in a biological pathway
- for the disease/condition in which they were first associated
all with no clear association with the particular phenotype
what is the longer name for gene E?
MCR1
what is gene E? (2)
- the extension gene
- determines if horse is red or black
what is dominant and recessive for gene E?
black is dominant (E)
red is recessive (ee)
does gene E determine pattern or placement of black pigment on a horse?
no; that is up to other genes
as controlled by genes OTHER than E, where can black hairs be found on a horse? (2)
- either in a distinctive pattern on the points (legs, mane, tail)
- or all over the entire body
what is the phenotype of EE or Ee?
black pigment in hair and skin
what is the phenotype of ee?
horse has black pigment in skin, but hair pigment appears red
describe the range of coat colors from ee (3)
- some shade of red, with NO black points
- ranging from liver chestnut to dark chestnut to chesnut or sorrel
- may have mane and tail that are dark red or brown
describe EE (4)
- no red factor detected
- cannot have red foals, regardless of the color of the mate
- base color is black, bay, or brown
- includes buckskin, zebra dun, grullo, perlino, gray, tobiano, overo, roan, or appaloosa
describe Ee (4)
- both black and red factoes detected
- can transmit either E or e to offspring
- base color of the horse is black, bay, or brown
- includes buckskin, zebra dun, grullo, perlino, gray, tobiano, overo, roan, or appaloosa
describe ee (3)
- only red factor detected
- base color is sorrel or chestnut (same thing = red)
- includes palomino, red dun, gray, cremello, tobiano, overo, roan, or appaloosa
describe the manes and tails of ee horses
manes and tails may be lighter (flaxen), or darker (BUT NOT BLACK), or could just be same color as the rest of the body
describe the confusing shit about chestnut and sorrel
they are the same color: red; just used in different areas;
AQHA says sorrel, byt thoroughbred people say chestnut
if you mate two ee horses, what color horse will you get?
only some shade of red because offspring will have to be ee
what is the longer name for gene A?
agouti signaling protein/agouti gene
what does gene A determine?
distribution of black pigmented hair
what does a dominant A in combination with dominant E yield?
will confine the black hair to the points to produce a BAY
describe the various shades of bay
range from dark bay (brown) through mahogany, blood, copper, and light bay
what are the 4 varations of the agouti gene? list in order of most to least dominant
- A+
- A
- At
- a
what must a horse have in its genotype in order to be a bay?
MUST have a dominant E and a dominant A
does the recessive agouti allele restrict the distribution of black hair? give what genotypes yield a uniformly black horse
no; because (aa) in the presence of the E allele produces a uniformly black horse
black: EE or Ee aa
describe A+ (3)
- wild bay
- black points of horse (legs) are not as extensive and appear washed out
- legs may have brown or silder appearing interspersed with black spots
describe A, give a personal example (2)
- traditional bay (Haida)
- black on legs extending to knee
describe At (3)
- dark brown (seal brown), but still genetically bay
- does not restrict black pigment as extensively as A+, but black is still washed out
- distinguished from black horse by brown hair around the muzzle and on flanks
what is the E and A genotype for a red horse? say why
ee_ _ (both _ could be A or a);
neither A nor a affects the pigment or its distribution in red (ee) horses, so it is not possible to tell by examination of coat color which alleles of the A gene a red horse has
describe A/A or A/a (3)
- black pigment distributed in a point pattern
- the horse will be BAY or BROWN in the ABSENCE of other modifying genes
- A has no effect on red pigment if (ee)
describe aa (3)
- only recessive allele is detected
- black pigment is distributed uniformly
- the horse will be BLACK in absence of other modifying genes
what are 2 modifications to base coat color?
- color dilutions to decrease the intensity of pigment
- white spotting or patterning
what is gene C? (2)
- cream gene
- pigment dilution
describe C^cr (2)
- incomplete dominance, affects color wothout a complete color change
- causes pigment dilution (of the red pigment)
what are fully pigmented horses’ genotypes for gene C?
CC (no dilution)
what happens to a bay (A_E_) when diluted with C^cr?; give
becomes a buckskin by dilution of the red body color to yellow WITHOUT affecting the black color of the mane and tail
give genetic formula for buckskin
A_E_CC^cr
what happens to a red horse (ee_ _) when diluted with gene C?
becomes a palomino through dilution of the red pigment in the body to yellow AND dilution of the mane and tail to flaxen/lighter
what is the genetic formula for a palomino horse?
ee _ _ CC^cr (could have any varation of AA Aa or aa because started from red horse)
can a genetically black horse (E_ aa) carry dilution?
yes, but not as noticeable
what does gene C^cr do in the homozygous condition? how do you get it? (3)
- completely dilutes coat color (double dilutes)
- results in very pale cream with white skin and blue eyes- cremello, perlino
- this is the product of mating two dilute-colored animals such as palominos or buckskins
what was a cremello before dilution?
was red (ee _ _)
what was a perlino before dilution?
was bay (E_ A_)
what was a smoky cream before dilution?
was black EE aa or Ee aa
describe CC (2)
- non-dilute
- basic colors are chestnut, bay, black, or brown in the absence of other color modifying genes`
describe CC^cr (4)
- heterozygous dilute
- chestnut diluted to palomino
- bay diluted to buckskin
- black diluted to smoky black
describe C^crC^cr (4)
- double dilute
- chestnut diluted to cremello
- bay diluted to perlino
- black diluted to smoky cream
what is gene D known for?
producing a dun patter and pigment dilution
is DD distinguishable from Dd?
nope
what are 3 differences of the effects of gene D versus C^cr?
- D dilutes both red and black pigments on the BODY, but does not dilute either pigment on the points (C^cr dilutes only red pigment and can affect the points)
- gene D resuslts in the presence of a particular pattern which can include dark points, a dorsal stripe, a shoulder stripe, and/or leg barring
- homozygosity for gene D does not produce the extreme dilution to cream as C^cr does
is gene D a dominant gene?
yes
what does the presence of at least one dominant D gene do to a red body color? (3)
can dilute it to a pinky-red, yellowish-red, or yellow
what does the presence of at least one dominant D gene do to a black body color?
dilutes it to mouse gray
describe leg barring as observed with the D gene
zebra stripes on legs
describe a red horse with the D allele, give genotype
the D allele results in a pinkish red horse with dark points, know as red dun or claybank dun; genotype is ee _ _ CC D_ (don’t know agouti or second D allele)
describe a bay horse with the D allele, give genotype
the D allele results in a yellow or yellow-ish red horse with blackpoints known as dun or buckskin dun; genotype is E_ A_ CC D_
describe a black horse with the D allele, give genotype
the D allele results in a mouse gray color with black points known as a mouse dun or grulla; genotype is E_ aa CC D_
in what two genotypes of horses can the effects of D and C^cr be easily confused? can a horse have both D and C^cr? if so, how do you distinguish?
A_ E_; can have both; D is only found in a few breeds of horses; in the USA it is only found in some pony and stock horse (quarter horse) breeds
give the letters of the silver dapple (taffy) gene
Z is dominant and silver
N is normal and recessive
how is the silver gene different from D and C^cr?
it acts on black pigment only
what happens to the black on the body in the presence of the silver gene? what happens to mane and tail?
the black on the body is diluted to a darm cream/light chocolate and the mane and tail are diluted or white
give the genotypes and corresponding effects of the silver gene
ZZ: dilution to a greater effect
ZN: dilution
NN: normal; no dilution
in what 3 breeds of horses is the silver gene found?
- rocky mountain horses
- icelandics
- morgans
what is an example of pleiotropy with the silver gene?
this gene can dilute black pigment but can also reuslt in malformation in the eyes, with ZZ having more severe malformation
does the silver gene affect red horses?
nope
give the letters of the champagne gene
Ch is dominant and champagne
N is normal and nonchampagne
describe a red horse (ee) with a champagne allele (Ch) (3)
- is difficult to distinguish from cream horses but
- Ch results in gold color with flaxen mane and tail
- these are called golden and ivory champagne horses
describe a bay horse (E_ A_) with a Ch allele
becomes amber champagne
describe a black horse (EE aa) with a Ch allele
classic champagne
describe the skin of Ch horses
has a pink hue that becomes speckled with age (speckles on the eyes, muzzle, and under the tail)
describe the eye color of a champagne horse
blue at birth, then changing to green or amber
what are 5 breeds that commonly carry the champagne allele?
- quarter horse
- tennessee walking horse
- saddlebred
- missouri
- fox trotter
describe the roan (Rn) gene (5)
- Rn is dominant
- mixture of individual white and colored hair in animals
- head and points will be colored hair
- non-progressive pattern: born with it and remains that same over the course of life
- amount of roan may vary with season as white and roan hairs grow differently
describe the gray gene (G) (2)
- exclusion of pigment from hair
- progressive depigmentation, so hair gets more gray with age
describe a horse with the gray allele at birth
will be born any color but gray and gradually become white or “flea-bitten” as an aged animal
describe a “flea bitten” coat color
white with red or black flecks
give the genotypes and corresponding phenotype of the gray allele
GG: gray
Gg: gray
gg: non-gray
what is the earliest indication of a horse with the G allele changing to gray?
gray hairs seen around the eyes
describe the intermediate stages of the graying process for horses with a (G) allele
horses will have a mixture of white and dark hairs (BUT NOT ROAN); two general patterns are dapple and flea-bitten
how long does it take for a horse with the (G) allele to turn from color to gray?
6-8 years
what is a bad thing often observed in gray horses? how common is it and what is the only good point?
high incidence of dermal melanomas around the tail and head; over 70% of grat horses older than 15 years will develop melanoma but they have lower malignancy
even though they are less malignant, what is tricky about the dermal melanomas observed in over 70% of gray horses older than 15?
if in a tricky area like the jaw or the anus it can be an issue
describe white patterning, generally (4)
- dominant white variants cause white markings but not necessaarily across the entire body
- some variants are lethal in the homozygous state
- there are more than 24 mutations associated with white markings
- horses will have pink skin! so watch for sun!!
describe the tobiano white spotting pattern (3)
- variable restricted pattern of white hair with underlying pink skin which can occur with any coat color
- pattern is present at birth and stable throughout life
- looks like paint was poured on top of a solid-colored horse; white crosses topline! head usually a solid color
give the genotypes and associated phenotypes with tobiano coat color
TT: tobiano
Tt: tobiano
tt: solid coat color
what are the 2 types of paints?
tobiano and overo
describe the distribution of white in a tobiano
white extends from the neck crest, withers, and croup in an apparent top-to-bottom distribution on the whole body
describe the legs, head, and tail of a tobiano
legs are usually white, the head is usually dark except for face markings, and there is white in the tail
what can a homozygous dominant tobiano (TT) sometimes look like?
might have color spots mixed in the white areas
list the genotypes and corresponding phenotypes of overo
O is dominant so
OO: overo
ON: overo
NN: normal
which is more common in the US: tobiano or overo?
tobiano is more common than overo
describe the white areas of an overo (5)
- white areas usually display more ragged edges than tobiano
- white DOES NOT extend over topline in overo
- white is horizontally arranged with pattern on body
- white patches commonly appeaar in middle of sides and neck
- white may appear on belly
describe the back and head of an overo
colored back, often called the frame because it “frames” the white; head usually marked excessively with white
describe the eyes of overos
sometimes blue
describe the legs of overo horses
will have at least one dark leg
is OO a good genotype to have? why or why not?
NO!! OO is lethal white foal syndrome; comes from crossing two overo horses (25% chance) and causes aganglionosis of the GI tract, where the lower part/large intestine develops without proper innervation so the foal cannot defecate and will die painfully
is sabino a particular gene?
no; can refer to a variety of white markings
describe Sb1
a specific sabino mutation that results in irregular white markings diffuse throughout the body
describe a horse with one copy of the Sb1 allele (3)
- horse will have 2 or mroe white legs
- will have white markings, often with irregular edges
- can cause roaning on legs and lower body
describe what is commonly referred to as a sabino horse (3)
- have a blaze
- have white on the lower lip
- have body spots on the mid-section
describe a horse with 2 copies of the Sb1 gene
primarily white
describe the relationship between clydesdales and sabino coat color
cyldesdales are called sabinos but DO NOT have the Sb1 gene; likely carry some other sabino variant
what genes cause the splash white coat color?
multiple alleles on two different loci:
SW1, SW3, SW5 and N (loci MITF)
SW2, SW4, and N (loci PAX3)
MITF and PAX3 are associated woth melanocytes so affect pigmentation
describe the dominance of the splash white alleles
most variants are dominant over N
describe the expressivity of the splash white alleles
variable expressivity, so one gene has a range of phenotypes
describe the effects of the SW1 splash white gene
additive; so one copy has SOME effect on genotype and two copies have additional impact (even more white)
describe splashed white horses, typically (5)
- white legs with body white ventrally
- head is extensively white (some exceptions)
- crisp edges between white and colored areas
- blue eyes are common
- sometimes resembles sabino
how are splash white horses described?
as if horses were dipped in white paint
what is the downside of the genotypes associated with splash white horses?
SW2, SW3, SW4, and SW4(SW5) may be homozygous lethal
in what breeds is the splash white coat color usually found? (4)
- quarter horses
- icelandic
- paint
- trakehner
what alleles are associated with appaloosa coat color?
Lp and PATN1
describe the Lp gene
also called leopard complex spotting locus; leads to a white coat pattern characterized by variable patterning with or without pigmented spots known as leopard spots (determines the presence of appaloosa pattern)
describe the PATN1 gene
modifier of Lp in appaloosas; controls the amount of white in the coat, increasing the amount of white present in horses that also have the Lp mutation (determines what the appaloosa pattern looks like)
describe the leopard complex associated with appaloosas (4) what are these horses also called as they age?
- mottled skin
- striped hooves
- white sclera
- progressive loss of pigment in the coat (called roaning but NOT ROAN) with age
also called varnish roan once this loss of pigment begins
describe a horse that is homozygous for leopard complex (LP/LP)
minimal to extensive white pattern with few to no pigemented spots; size of the pattern is dependent on the modifiers present
what are 2 downsides observed in horses homozygous for the leopard complex (LP/LP)?
- congenital stationary night blindness (CSNB)
- elevated risk for equine recurrent uveitis (ERU)
describe a horse with the LP/N genotype
variable white pattern with pigmented spots; size of white pattern depends on what patterning modifiers are also present
describe PATN1 (4)
- appaloosa pattern-1
- modifers of LP
- controls the amount of white in the coat
- increasing the amount of white present in horses that also have LP mutation
describe N/PATN1 horses that are also LP (N/LP or LP/LP)
will have high amounts of white patterning; typically more than 60% at birth
describe PATN1/PATN1 horses that also have at least one copy of LP
have high amounts of white patterning (almost white looking)
