NOTES AND COMMENTS

Evening Post, Volume CXI, Issue 72, 26 March 1931, Page 10

NOTES AND COMMENTS

BY "KAN*

COLOUR OF HORSES

RULES OF INHERITANCE

THE MENDELIAN PRINCIPLE

Though the horse is the medium of so much in the way of a venture, Jvature herself does not gamble upon the colour scheme of the thoroughbreds she turns out. Whether the foal will be bay, brown, or chestnut, and to a somewhat less certain extent black or grey, is all decided upon a set rule of colour dominants. Accidents can happen, but then only by a freak. The Mendel system provides the theory of this colour breeding, nnd the following article offers an interesting and simple explanation of its workings. Among the majority of stockbreeders it is almost universally believed that the result of the mating of two whole colours | is purely a lottery. This may be correct to a certain extent, for it is true that we cannot define definitely to which colour the progeny will throw; but we do know ■what result we may expect from the union of two whole colours. To a monk, Gregor Mandel, credit must be given for expounding the theory of certain inherited characters which aro: handed down from parent to -offspring. In his monastery garden at Brunn (Austria) he crossed varieties of edible pea. He found that a tall pea crossed with a' dwarf pea gave 100 "per cent, tall peas. It - will be seen that the tall peas prevailed; over the dwarf ones. jMendcl, therefore,, named them "dominant," and the dwarf' ones he called "recessive." Now, if the crossbred tall peas are bred together, the result will be 75 per cent, tall and 25 per cent, dwarf. We then come to a remarkable conclusion: if the 75 per cent, (tall) are bred together, the offspring will still continue to be in the proportion of four to one, but, if the 25 per cent, (dwarf) are bred together, all the offspring will be dwarf, and no matter how many generations tne experiment is carried on they will show no signs of ever having been crossed. . Mendel was a botanist and not a biologist, and it was not. until about sixteen years after his death in 1884 that the significance of his sxperiments wasapplied to biology. APPLIED TO HORSES. • Let us now apply Mendelism to the inheritance of coat colour in horses. It must first be understood that the inheritance of.colour (except black and grey) is derived from two factors in each parent; of these two factors the offspring takes one from each parent. For chestnut we will call the factors C. and C, and bay or. brown B. and B. Thus mating two pure bay or browns together we get 100 per cent, bay or brown (BB). Since the stock used were pure in colour, it is obvious that we could get nothing else than pure offspring. Now, mating a pure bay or brown and a chestnut, we get 100 per cent, bay or brown crossbreds (BC). Just as Mendel, found that tall peas were dominant over the dwarf ones, we find that bay or brown is dominant over chestnut, and as the offspring takes one factor from each parent, we get 100 per cent, impure dominants (that is, they are crossbreds in make-up, yet bay or brown in colour). - Now let its mate two impure dominants, BC and BC, and the result is found to be, on an average in four matings, BB (pure brown or bay), BC (impure brown or bay), CB (another impure brown or bay), and CC (chestnut). This means that 75 per cent, of the offspring can be expected to be bay or brown and 25 per cent, chestnut. • Three of four offspring, on the general average, having a bay or brown factor in their make-up, will show bay or brown in colour, while the fourth (CC), having no bay or brown factor, will show chestnut. The first (BB), although the resultant mating of crossbreds, will bo a pure dominant, for he has no other factor in his make-up except, bay or brown. The next two (BC and CB) will be impure dominants (bay .or. brown in colour, yet having both factors), and the last (CG) will be chestnut showing the .recessive character."- "■ To I.ho keen observer, it will be seen that the so-called weight of ancestry has nothing at all to do with the inheritance of coat colour. ■ ■ , THE THIRD GENERATION. In the third generation, after a pure bay or brown has been mated with a chestnut, the expectation is that there ■will be, in every four offspring,.one pure bay or brown (BB). one chestnut (CC), and two hybrids (BC and CB) appearing ; as bay or brown. It will be seen that, although one of the grandparents was a chestnut, there is among the grandchildren, on a general average in every four, a pure dominant having no chestnutfactor in his make-up. This animal is just as irfi. from chestnut colour as the bay or brown in the original mating. Of course, at casual observance it is not possible to say whether a bay or brown is pure dominant or hybrid, and the point can be tested only by breeding. Before Mendelism opened up this avenue of'investigation, it was observed that certain thor-oughbred-horses never sired chestnuts. Sunrise, by Robinson Crusoe out bf Sunshine, by Maribyrnong, is an acknowledged case, for although he was mated with chestnut mares he never threw to the lighter colour. It would be almost certain, therefore, that he was a pure dominant. . It. was stated earlier that the principle determining the inheritance of colour did not" apparently apply to black or grey. The transmission of the black coat colour is most uncertain, nnd docs not conform to any. known rules, except that black is often dominated by other colours and so kept latent. Indeed, ilie factor for transmitting black frequently lies latent for many cenerntion's. and then crops up unexpectedly and becomes dominant in certain individuals. This was exemplified in the case of the bay mare. Tradition, who produced many foals of different colours to various sires, many of her progeny being black. Tradition had many lines tracing back to the black horse Trumpator (1782), who was by Conductor (chestnut) from Brunette (brown). Trumpator himself had no black forbears for many generations, but had eight crosses of the Byerly Turk to account, for his colour and for his power of transmitting it. The black coat is apparently always traceable back to the influence of the Byerly Turk. C4rey also is not inherited on the same principle as bay or brown, or chestnut. This colour is due to ths absence of pigment in the fibres of the hair. A grey must have at least one grey parent. Should it be possible to remove the structure that prevents the flow of pigment into the hairs, then the animal would become whole coloured at once-^-bay, brown, bkcK, or chestnut, according to the factors inherited from its parents.