The fundamental law of inheritance which underlies character recombination in new breeds, and to which also established breeds owe their uniformity, is known as Mendel's law (q.v.). This is the law of unit character in heritance, in accordance with which the dif ferent inherited characters are found to behave like independent units in transmission and so can be rearranged in new combinations. Each individual of an established breed of animals transmits a certain assemblage of inheritance units, which constitute the breed characteristics. So long as matings are made only within the breed all the progeny have these same inherited breed characteristics, such as black color and hqrnlessness in the case of Aberdeen-Angus cattle, red color and hornlessness in the case of Red Polled cattle, red color and horns in the case of Devon cattle, and white face and horns in the case of Hereford cattle.
But if we cross a breed of black cattle with a breed of red cattle, the first generation off spring will be black, the color unit black being stronger than the color unit red. We call black the dominant unit of the two, and red the re cessive unit. But the black crossbreds have inherited red no less than black, even though they do not show it, and they will transmit it to half their progeny the alternative with black being transmitted to the other half of the progeny. Accordingly if a crossbred black bull is mated with red cows, we find that on the average half the calves are black in color' and half are red. This comes about as follows: The crossbred bull' transmits the black unit to half his calves. This unit meets a red unit transmitted by the red dam and dominates it producing a black calf. To the remaining calves, half of the total, the bull transmits the red unit which meets a red unit transmitted by the red dam and thus produces a pure red combination. Such reds will breed true because they contain only the red unit. But the blacks which contain also a red unit will not breed true. Nevertheless true breeding blacks may be obtained even from crossbreds, by mating black with black. If the crossbred black bull were mated with pure black cows, half his calves would be pure blacks, viz., those to which he transmitted the black unit, for to all the calves in such matings the black dam would transmit the black unit. If the crossbred bull were mated with crossbred cows, one fourth of the calves would be pure blacks, two fourths would be impure blacks (not true breeding) and one-fourth would be pure reds. For each parent would transmit impartially either the black or the red unit and the ex pected combinations would be: Units transmitted by sire, B or R Units transmitted by dam, B or R Combinations BB, BR, BR, R R Pure black Impure black Pure Red If the two breeds of cattle which we crossed were Aberdeen-Angus and Red Polled, all the cross-bred calves would be polled, since both breeds possess this same unit. 13ut if we crossed Aberdeen-Angus cattle, which are black and polled, with Devon cattle, which are red but have horns, we should get a different re sult, viz., imperfectly developed horns (scurs) in the crossbreeds, which would all be black. A crossbred bull of this sort would transmit to half his calves the polled condition and to half he would transmit horns. To half he would transmit black color and to half he would transmit red color_ But horns and color are distinct units in heredity, hence the com binations of units which a crossbred of this sort might transmit would be of four sorts, viz., polled black, polled red, horned black and horned red. In fact animals presenting these four sorts of combinations would be obtained in the second generation of crossbreds pro duced by mating first generation crossbreds with each other. This case illustrates how the principle of unit character inheritance may be utilized to secure new combinations of inheri tance units in which breeds differ. The maxi mum number of ,recombinations may be ob tained in the second crossbred, not the first crossbred, generation. This principle has un
doubtedly been utilized, consciously or uncon sciously, in producing existing breeds and can be used more freely in the future because its operations are now better understood. Never theless the crossing of breeds is something which should be undertaken cautionsly, for it will be found that few cases are as simple as the hypothetical one just discussed. Most breeds differ by a great many unit characters and a cross in such cases produces an extrem ely variable second generation, because of the large number of recombinations of characters which will be obtained, and few of these re combinations, even if desirable, can be isolated in true breeding types without subsequent selec tion through several generations.
Moreover most inherited tharitters are not simple (behaving as single units) but depending upon the simultaneous presence of several or many' independent units. This makes their manipulation in crosses extremely difficult. Differences in size, early maturity and produc tiveness are among the most valued economic characters of breeds, yet these in no case behave as simple units in transmission. They com monly behave neither as• dominant nor as re cessive units but yield intermediates or blendS. A second crossbred generation again 'yields blends but with increased variability, giving indications of segregation more or less com plete. This is interpreted as meaning that many distinct units are involved - which segre gate and recombine independently of each other. In such cases crosses are useful only if the production of intermediates is desired. If it is desired to extend the ranee of existing vari ation in quatititatiN e characters, it is not to cross breeds, hut to practise systematic selec tion in the desired direction within the breed which most nearly approaches the ideal. For example, suppose increased quantity production of milk is desired in cattle. The most produc tive of existing breeds is admitted to be the Holstein-Friesian. If this breed is crossed a less productive breed, the offspring are found to be of intermediate productiveness. Such a cross would not be a promising means of obtaining productiveness greater than that of the Holstein-Friesian. It would be better to search within the breed for families of unusual productiveness and try further to improve these best families by selection, avoiding close in breeding however by mating with each other representatives of different families of great excellence. This is the practise now followed by the best informed breeders. Still there are conditions under which crossbreeding, even of Holstein-Friesian cattle, may be desirable. Holstein milk contains too little butter fat to satisfy some markets. Improvement in its quality may be secured by a cross with dairy breeds which produce richer milk, though less of it as is true of Guernseys and jerseys. Whether the improvement in quality thus se cured will offset the diminished quantity pro duction entailed by the cross and the undesirable variability of the later crossbred generations is a matter for experimental determination.
A fifth maxim for the breeder may now be formulated. (5) Both the uniform excellence of established breeds and the making of new breeds with novel 'combinations of characters depend upon Menders law of unit character inheritance. The breeder should be thoroughly familiar with the workings of fundamental late.
Bibliography.— Babcock, E. B., and Clau sen, R. in Relation to Agricul ture' (New York 1918) ; Darwin, Chas., 'Vari ation of Animals and Plants under Domesti cation) (New York 1871) ; Gay, C. W., (The Breeds of Live Stock) (New York 1916) ; Keller, C., 'Naturgeschichte der Haustiere' (Berlin 1905); Plumb, C. S., 'T es and Breeds of Farm Animals' (Boston 1907) ; Wallace, Robert, Live Stock of Great Britain) (Edinburgh 1907).