6. W = dominant white, dark eye w = non-white complete dominance with regard to color( WW, lethal when homozygous?)
7. Wm = dark or pigmented mane wm- light or flaxen mane complete dominance
8. I = intense pigment; chestnut? i = dilute pigment complete dominance( may be more than a single pair of alleles involved).
The basic genotypes of the various coat colors in horses are as follows:
1. Black: BB( or Bb), ee, gg, ro ro, dd, ww
2. Bay: BB( or Bb), EE( or Ee), gg, ro ro, dd, ww
3. Chestnut: bb, EE( or Ee)( or ee), gg, ro ro, dd, ww
( flaxen mane and tail, wm wm) II( or li) = dark or intense color ii = light red or sorrel color.
4. Brown: The genetics of the brown color in horses is seemingly quite complicated. Castle and Singleton proposed that black, brown, and bay are influenced by two allelic series. There is one basic gene for brown, e’, which is allelic to both E and e. The other allelic series of three genes determines the intensity of dark pigmentation.
5. Gray: BB or Bb, EE( or Ee)( or ee), GG( or Gg), ro ro, dd, ww. G is epistatic to all other basic color genes, B or b, E, or e, and etc. except perhaps D and W.
6. Roan: Occurs most commonly in sorrel or chestnut horses, bb, EE( or Ee)( or ee), gg, Ro Ro,( or Ro ro), dd, ww.( Strawberry roan).
7. Palomino: bb, EE( or Ee)( or ee), gg, ro ro, Dd, ww( Preferably wm wm, for lighter mane and tail)( Action of D upon basic chestnut color).
8. Buckskin or coyote dun: BB( or Bb), EE( or Ee), gg, ro ro, Dd, ww( Action of D upon basic bay color. Most duns of a red color with typical black points are comparable to this genotype. Perhaps the gene, i, causes some variation in intensity. If D acts upon the basic black color, the dark, or mouse, dun color is probably produced).
9. " Albino ":( Pseudo, with blue eyes). A horse homozygous for D,( DD), is a pseudo albino. The coat color may vary from a light cream color to almost white. The skin is pink and the eyes are blue.
10. " Albino ": White with dark eyes. A horse possessing W will be white and dark eyed regardless of other genes for color, except perhaps for individuals homozygous for D, In this case they would probably be blue eyed. It has been suggested that individuals homozygous for W do not survive because of a lethal effect of the gene. According to this theory, these white horses would necessarily be Ww or heterozygous, and not WW.
11. White Spotting: Restricted white spotting( white feet, white stockings, and white markings on the head) appear to be transmitted as a general recessive genetic factor. Extensive white spotting, as in pintos and paints, appears to be due to a single dominant gene. Horses of this description may be homozygous or heterozygous.
White pigmentation of the iris of the eye may occur in horses. This condition is commonly termed " glass eyes." Vision is apparently unimpaired. There is no evidence to indicate that glass eyes are caused by a separate or individual gene for that character alone. Glass eyes occur most frequently in horses possessing large amounts of white spotting on the head, such as the " bald face," and are probably the result of the action of the same genes.
The material presented here indicates that many genes influence coat color in horses. A horse ' s phenotype provides partial information on genetic composition; however an individual may possess and transmit many genes not revealed by phenotype. These are primarily the recessive genes which are masked by complete dominance under condi-, tions of genetic heterozygosity. Horse breeders can rather easily obtain additional information on the genotypes of breeding stock by observing the results or traits of their offspring. Parents possessing the dominant phenotype are identified as heterozygotes when they sire or produce colts having the recessive phenotype.
UNDESIRABLE TRAITS IN HORSES
Undesirable traits occur in all categories of characteristics in farm livestock. This includes coat color, structural tissues, temperament, physiological systems, physical performance, longevity, reproductive performance and others. Many, or even most, individual gene effects upon these traits are of such limited magnitude that they are not identifiable. Others are definitely recognizable because their effects are large and may interfere with survival. It would indeed be unusual if there should be an animal that possessed no undesirable genes whatsoever. Individual undesirable genes are usually not of major concern unless their effects are large.
The subject of coat color inheritance has been discussed. Preference or discrimination with regard to color characteristics depends largely on breeder or owner preference. This is an area, however, in which individual gene effects are large and hereditary influences are free from environmental influence.
Traits among livestock that impair survivability are especially objectionable. Those destroying life embryonically, or at the time of birth( perhaps shortly thereafter), are known as lethals. Other objectionable traits that cause death prematurely at any time in life are called sublethals. Many of these conditions are caused by undesirable genes. An animal that possesses and transmits a lethal or sublethal gene is a detriment to his species. Most lethal
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