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
( Continued , on Page 60 )
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