Heredity, race, and society.

  • Dunn, L. C. (Leslie Clarence), 1893-1974.
Date:
1947
Catalogue details

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

Credit: Heredity, race, and society. Source: Wellcome Collection.

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    creased. This is of great importance in practical agricul ture. Suppose that you have two varieties of a crop or vegetable plant, and that each variety possesses an ad vantageous trait which the other does not have. For example, one variety may yield well but may not have sufficient resistance to frost, while the other is frost resistant but produces unsatisfactory yields. It is obvi ously desirable to have a combination of good yield and frost resistance in the same plants. If yield and resist ance are chiefly influenced by two pairs of independent genes, then according to Mendel's second law, this may often be accomplished by crossing the two varieties, and selecting in the second generation of hybrids the plants which possess the desired combination of traits. Plants in which the undesirable qualities of the initial varieties are combined will also appear among the hybrids, mak ing a careful examination of individual plants and their progenies essential. Many of the best varieties of cultivated plants have been arrived at by combining through appropriate crosses the desirable traits which were present separately in several older varieties. Causes of Variety Independent inheritance greatly increases the num bers of different kinds of individuals. Mendel showed that when two races of plants differed by three inde pendent characters, such as one with yellow round seeds and purple flowers and another with green wrinkled seeds and white flowers, the descendants of a cross be tween these varieties were of eight different types, i.e., each of the four combinations of yellow, round, green and wrinkled could be either purple or white, which doubled the numbers of combinations. In fact every additional pair of independent character differences which show dominance doubles the number of combi nations. If in a human family we have differences such as (1) taster or taste-blind; (2) dark or light eyes; (3) short-fingered or normal; we have 8 possible combina tions. As anyone can tell by looking about him, the