Heredity / by J.A.S. Watson.

  • Watson, James A. S. (James Anderson Scott), 1889-1966.
Date:
[1912]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Heredity / by J.A.S. Watson. Source: Wellcome Collection.

    88/104 (page 86)
    86 HEREDITY fulness of vigour appears to vary considerably in different species, and in the same species under different con¬ ditions. The earlier breeders of Shorthorns, working, as they were, with foundation stock of very mixed character, were accustomed to mate together very closely related individuals—sire and daughter, half-brother and sister, and the like, generation after generation. On the other hand, it is safe to say that few turf winners have been produced by matings of animals more closely related than half-cousins. In any case, continued close inbreeding invariably leads to greatly weakened con¬ stitution, and often to partial or complete sterihty. We may assume with some degree of certainty that this is due to the reduction of many characters to the pure condition—to the formation of pure lines, in fact— for the loss of vigour occurs in the same manner as in plants. Whether these pure lines can actually exist we do not know, but in some animals at least, sterility seems to result before this stage is reached. It is evident that where a certain amount of inbreed¬ ing is practised, there will be less opportunity of varia¬ tion within the strain. For the number of hybrid characters is reduced, and the type is thus partially fixed. Thus a certain amount of inbreeding introduces an element of certainty into the work of the breeder. The mating of almost entirely unrelated individuals gives, on the average, the most vigorous type of off¬ spring. But it is evident that if this method of breed¬ ing is continued, the resulting offspring wiU. tend to be very irregular, for there are chances of widely different combinations of factors. Breed ' in ' to fix type ; breed ' out * to secure vigour ; in general, compromise —this may be said to be a summing up of the experience of animal breeders. Theory may be said to have caught up with practice by suggesting reasons for these rules, but it can offer little in addition. One might perhaps prophesy an extension of the present practice of systematic crossing in order to secure both vigour and uniformity, but there are difficulties in the way of such systems of breeding.