NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/rec/equestri/6956

Internet Message Format | 1993-01-23 | 15.3 KB

Path: sparky!uunet!paladin.american.edu!howland.reston.ans.net!usc!cs.utexas.edu!asuvax!ncar!noao!amethyst!organpipe.uug.arizona.edu!news
From: [e-mail] (Tracy Scheinkman)
Newsgroups: rec.equestrian
Subject: re: horse color genetics program
Message-ID: <[e-mail]>
Date: 22 Jan 93 20:35:43 GMT
Sender: [e-mail]
Organization: University of Arizona, Tucson, AZ
Lines: 259


    Since this subject has come up again I thought I'd post the info
that I have.
    First, color inheritance in horses is NOT governed by a single gene.
Hair color of horses like hair color of humans and other mammals is governed
by many genes interacting with each other.
    To a certain extent it can be thought of as a series of transparent
overlays with a figure of a horse underneath, what color the horse is
depends on which overlay is uppermost and how much of the underlying colors
it allows to show through.  For example in the case of a horse which has
both a gene for roan R and a gene for grey G, both of which are dominant
genes, you will not be able to tell that the horse has a roan gene because
the greying covers it up.  The only hint you would have would be when the
foal is just born if their coat is about half white hairs mixed evenly with
the backround color, except at the head where roans do not have as many
white hairs as greys, then you would know that the foal carried roan coloring
but shortly thereafter the foal's coat would begin to grey out as the grey
gene is a progressive whitener of the coat.  Thus as an adult the horse could
conceivably carry a roan gene and yet look perfectly white.
    Now on to the next lesson.  Genes always come in pairs.  Geneticists
label them with a capital letter if the gene is dominant or a small letter
if recessive thus G represents grey color and is dominant, g represents non-
grey and is recessive.  In order to see a recessive color both genes in a
pair must be recessive thus a bay horse would have gg at the grey gene
location (called a locus) and a grey horse would either be GG or Gg.  Got it?
    Next, not all books use the same lettering system for different
genes a gene called A in one book might be B in another.  Just look for
consistency within the book or article itself.  The book I will be referring
to is called Horse Color and was written by D. Phillip Sponenberg,Phd.,DVM and
Bonnie V. Beaver, DVM.  Sponenberg is a researcher with a university in
Virginia, I believe, and is still doing research into horse color.  This is a
wonderfully complete book which includes over a hundred color photographs
of the various coat colors and patterns they discuss.  The appaloosa
information which is in the book is incorrect and Sponenberg has recently
published new information on appaloosa inheritance which I have tried to
include here (see Equus, April 1990 issue).

    Horses have the possibility of two different color pigments, eumelanin,
which is responsible for black and chocolate brown horses and the black in a
bay's mane and tail, and phaeomelanin, which is responsible for the red or
yellow color of sorrels, chestnuts, palominos, and the red body on clear
bays.  Now we're ready for the genes themselves.
    A    a dominant gene is responsible for bay horses by restricting
        eumelanin to the points meaning the mane, tail, and legs of the
        horse, the rest of the horse has red, phaeomelanin, pigment
        (note the exception: dark mahogany bays and seal browns
        have other genetic elements at work, mahogany bays have an
        additional gene allowing some eumelanin, seal browns are
        actually genetically black with another gene P causing
        light areas on muzzle and flanks)
    a    this recessive gene is responsible for black and uniform
        chocolate brown horses, a common color for Morgans and
        some Quarter Horses, this gene allows eumelanin over the
        whole horse uniformly (as compared with A which restricts
        eumelanin to the points)
    B    this gene is for the black variety of eumelanin
    b    this recessive gene is for the brown variety of eumelanin, the
        difference apparently is in the microscopic arrangement
        of pigment molecules, these horses also tend to have
        amber or light brown eyes and pinkish brown skin, in order
        to have a uniform chocolate brown horse then the horse must
        be aabb, if it is A-bb then it is a bay with brown points
        instead of black points(red body, brown mane tail and legs)
        this gene is only a factor in a few breeds most notably
        Morgan and Quarter Horse
    C    this gene means that the horse's color is fully expressed,
         non-dilute
     cr
    c      this recessive gene is the cremello gene it dilutes phaeo-
        melanin markedly, eumelanin a little, it is responsible
        for blue-eyed light cream or white horses that some
        call Albinos (true name is cremello if chestnut is diluted,
        perlino if bay is diluted), however there is no true Albino
        gene for horses, this gene is also incompletely recessive so
        when big C and little c-cr are present in one gene pair a horse
        that would otherwise have been chestnut or bay would be
        instead palomino or buckskin respectively.  Fascinating, huh!
    D    a dominant dilution gene is responsible for all dun horses
        except claybank duns which are mostly c-cr horses, this gene
        dilutes body color but not point color, duns have
        dorsal stripes, some also have leg striping, black becomes
        grullo when D is present, chestnut becomes red dun, bays
        become line-backed buckskins, other examples of dun colors are
        lilac dun, olive grullo, line-backed palomino, zebra dun,
        yellow dun, etc. the dun gene D can act in concert with
        other dilution genes, for example with Cc-cr in the case of
        a chestnut D- would produce a line-backed palomino
    d    non-dun
    E    this dominant gene allows eumelanin at the points meaning
        it allows bay and black this allowance of black color
        becomes important because of the next gene
    e    this recessive gene causes phaeomelanin red or yellow
        over the WHOLE body and points of the horse in other words
        chestnut, sorrel, or palomino, this gene is said to be
        epistatic to the A locus this means that if two e genes are
        present they cover up the effects of A or a, think of it as
        opaque plastic overlays the horse might have been black, bay,
        or chocolate brown according to its other genes but because of
        ee it's red (note: sorrel and chestnut both generally refer to
        the same color genetically, red, however different breed
        associations refer to the lighter phases of the color
        differently than the darker phases of the color, to further
        complicate things different breed associations do not agree as
        to what term shall cover what shade of color)
     d
    E     the proof for this gene is incomplete, this is a dominant
        gene at the E locus that causes the color called jet black
        which is a non-sun-fading black color mostly seen on
        Clydesdales and a very few other large breeds it is not
        present for example in Arabians whose black color when
        present comes solely from the normal recessive mechanism
    F    normal red mane and tail on ee, chestnut or sorrel horses
    f    flaxen mane and tail on chestnut or sorrel horses
    G    grey, this dominant gene is like a transparent plastic
        overlay, when the foal is born whatever color it would
        have been without the G shows through, thus it is black,
        or chestnut or bay or whatever, then as it grows older
        it progressively whitens as each new coat gets more and
        more white hairs mixed into it
    g    non-grey
    P    pangare (pronounced pan-gar-ray), this causes light areas
        on muzzle, over eyes, on flanks, stomach, it causes
        black to become seal brown and chestnut with flaxen mane
        to become blond    sorrel
    p    non-pangare
    Sty    smutty, causes some black to become mixed into body coat
        clear sorrel becomes chestnut or liver chestnut, clear bay
        becomes mahogany bay, palomino becomes smutty palomino, etc.
    sty    recessive causes clear pure color, a clear pure red is
        often called sorrel among Thoroughbreds and Quarter Horses
        other breeds call this chestnut
    Z    silver dapple, causes eumelanin to be diluted to flaxen usually
        a little eumelanin remains in the coat, silver dapple bays are
        possible when the eumelanin in the mane and tail are diluted
        causing the mane and tail to have a silvery color to them
        because of some of the remaining black hairs, this gene is
        really only a factor for breeds such as Shetlands, Icelandics,
        Dutch Warmbloods, and Norwegian Fjords, Norwegian Fjords may
        have a combination of silver dapple and dun genes
    z    non-silver-dapple
    R    roan, causes white hairs to be mixed with base coat color,
        this color is non-progressive as opposed to grey which
        is a progressive whitener, though it does change a little
        with the seasons, roans often have fewer white hairs mixed in
        to the base coat on the face and legs than on the body, also
        this gene is thought to be a dominant lethal meaning that RR
        horses die during development, most roans are Rr and throw
        solid colored foals as well as roans, Dutch and Brabant draft
        horses may be an exception to this rule and if so roans in
        those breeds are probably due to some other mechanism
    r    non-roan
    T    tobiano paint spotting, this paint color has bold sharp edges
        and has white in a vertical pattern that often crosses the spine
        the face and legs are usually dark the amount of white is
        governed by independent modifiers and can be selected for,
        thus a tobiano with a lot of white will tend to have foals with
        a lot of white, homozygous tobianos, meaning TT horses, will
        throw 100% tobiano color
    t    non-tobiano
    O    non-overo
    o    overo paint spotting, also called frame overo, there is some
        argument over whether this gene is recessive or dominant, the
        color tends to splash in a horizontal pattern with sharp edges
        that rarely crosses the spine, the legs and face are often
        white, blue eyes are common, overos that are mostly or all
        white die within a few days of birth because of a malfunction
        of the colon, there may be another gene at this locus that is
        responsible for these lethal white overos, the amount of white
        caused by this recessive gene is governed by an unknown
        mechanism possibly womb temperature maybe independent genes
        but cannot be selected for, overos often throw solid or nearly
        solid foals
    Sb     sabino paint spotting, often confused with overo, this may be
        an example of incomplete dominance such that homozygous horses
        SbSb have more extreme patterning and markings than
        heterozygous horses Sbsb, this pattern is typified by extremely
        ragged margins rarely crosses the spine and blue eyes are
        common, the minimum expression seems to be high white
        stockings and extreme facial white, this gene is common amongst
        Clydesdales where the color is sometimes erroneously called
        roan, many sabino horses exhibit the Medicine Hat pattern
        popular among some breeds such as the North American Spanish,
        pure white foals develop normally in contrast with overos
    sb    non-sabino
    Note: the term tovero refers to a horse which exhibits a combination
        of overo and tobiano patterning, such horses would be
        genetically T-oo, a horse that is genetically T-ooSb is
        theoretically possible, either tovero or sabino horses may
        exhibit the Medicine Hat pattern
    Rb     rabicano, which is a white hairs starting at the dock of the
        horse's tail and sometimes white hairs mixed in the flank area
    rb    non-rabicano
    W    dominant white, neither this gene nor the c-cr genes are true
        albino genes as some pigment is still present, the skin of
        dominant white horses is pink, the eyes of such horses are
        usually brown, this is a dominant lethal gene meaning that all
        dominant white horses are Ww, the WW form apparently dies in
        development there are no exceptions
    w    non-white
    Apl    non-appaloosa, to be a non-appaloosa the horse must be
        Apl Apl
    apl    this is another incompletely recessive gene, apl apl horses
        are the few spot appaloosas that produce 100% appaloosa
        babies no matter who they are mated to, Apl apl horses are
        the brightly patterned appaloosas we are most familiar with,
        other modifier genes cause the different patterns -- leopard,
        blanket, varnish roan, etc. -- combination patterns are
        common, for example I once saw a black varnish roan with a
        white blanket over the rump and leaopard spots over the
        whole horse, there may be other appaloosa mechanisms that
        are unknown currently (note: the grey gene has the same affect
        on appaloosa pattern spots and splashes that it has on solid
        colored horses and eventually such appaloosas that carry the
        G grey gene will become completely white just as solid grey
        horses do, in those cases the skin under the white hairs is
        often visible and is pink in the blanket areas and dark where
        the spots or solid areas were, thus an all white horse with
        pink skin and a few oval spots of dark skin may actually be a
        few spot appaloosa that has turned grey) currently there are
        about eight different known appaloosa patterns, there may be
        a form of Appaloosa roaning (or greying) which only affects
        the backround color and does not affect the spots however
        evidence for this is not available

    Now how it works, let's take the case of a black horse, which is one
of the most difficult colors to achieve in most breeds.  Remember that
chestnut ee covers up black aa and bay A-.  So a chestnut horse with a black
ancestor is bred to a bay horse with a black ancestor, the chestnut's gene
pattern looks something like a?ee the bay's gene pattern looks something
like AaE?, now there are 16 different combinations possible of which 4 are
definitely bay, 4 are bay or chestnut depending on what genes the ? are,
4 are chestnut or black, 2 are bay or black, and only 2 are definitely black.
If we make the first ?=A and the second ?=e then, the possible offspring
are 8 chestnuts, 6 bays, and 2 blacks in other words a ratio of 4:3:1.   When
you add more color genes it becomes more complicated.
    As to the grey question, grey covers up the base color, let us say
that color is bay, well bay can hide a black gene or a chestnut gene, so
depending on what the grey is bred to you could have a chestnut, a bay, a
black, or another grey, or numerous other colors.  Remember to look at
your horse's breed and parentage, certain breeds don't have some color genes
available to them.  For example Arabians don't have Z silver dapple or E-d
jet black, or b chocolate brown so those genes are unnecessary to consider.
My mare's line has had nothing but greys, bays, and chestnuts for many
generations, black may have been known many generations ago but hasn't shown
up since then so I won't need to bother checking for other colors such as
overo genes or cremello genes when I breed her to another of her line.  Since
I am interested in black I would want to see if the sire had a line that
included black genes.  One more thing, the likelihood of a recessive gene
showing up in subsequent generations decreases with each new generation that
does not show it, but, as in the case of black coloring, it never completely
goes away.  If on the other hand you selectively breed away from a dominant
color it can be completely lost in one generation (that actually happened
with the Crabbet Arabians in the early 1900's, it wasn't until Skowronek
was found and purchased that the grey color reappeared in the Crabbet herd.)

                Tracy and Bruce (beautiful man) and
                Cachet (7 year old Arabian mare) and
                Mithril (2 year old Arabian filly,
                daughter of Cachet) and The Cats