Breeding methods for cattle, pigs, and poultry in the United States : including inter-state co-operation project no. 253.

  • European Productivity Agency.
Date:
[1957]
Catalogue details

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

Credit: Breeding methods for cattle, pigs, and poultry in the United States : including inter-state co-operation project no. 253. Source: Wellcome Collection.

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    simple gene pairs which could be readily analysed without any great effort of time or money. It is advisable to regard such work as forming but a small part of modern genetics and, in fact, to describe it as Mendelism. Utility performance of poultry and other animals, on the other hand, depends on large numbers of genes and therefore requires analysis by complicated sta¬ tistical methods. This more recent science is therefore known as Population Genetics. Its techniques are beyond the average man and indeed the average breeder ; but most agricultural students in the United States have to undergo a course of train¬ ing in its mysteries, from which they gain at least some useful, if rule-of-thumb, knowledge. The expert, however, must be a mathematician. Where any breeding project involving utility characters is envisaged, the mathematician is automatically and of necessity brought in at the very beginning to plan the experiment and to evaluate its results. It follows that much of the earlier work on the subject of poultry breeding has had to be rejected as insignificant, and a whole new series of experiments begun. Modern research into the new techniques of breeding based on preliminary inbreeding was thus initiated by Shull and East between 1905 and 1912. We cannot describe their work in this place, but the general subject as it concerns poultry must be given further discussion. INCROSSING, TOPCROSSING AND INCROSSBREEDING 117. If two inbred lines of the same breed or variety are mated together, the resulting progeny are called incrosses. Where the two inbred parent lines are of different breeds or varieties, the progeny are known as incrossbreds. If one inbred line is mated with outbred stock of the same or another breed or variety, the progeny are called topcrosses. It is important to bear these definitions in mind, otherwise confusion may result. Any combination of two inbred lines of poultry is called a two-way cross, and a combination of two two-way crosses produces a four-way cross. By mating an inbred line with a two-way cross, we get a three-way cross. 118. In the case of poultry, just as with corn, it was found that two-way crosses were sometimes, but by no means always, superios in performance not only to the inbred parents, but even to outbred stock maintained under the same environmental conditions. Some combinations of inbred lines were better than others, but performance could not be predicted from a know¬ ledge of the pure inbreds. Some latent productive force, in other words, can be released by this process, but the right combina¬ tions can be judged only by trial and error. One may describe the phenomenon as the release of cryptic gains acquired during inbreeding, or one may refer to what has been stated above about the removal of genetic trash, followed by the restoration of favourable gene complexes. But neither of these two straight- 44