Studies of Heredity in Rabbits, Rats, and Mice

Studies of Heredity in Rabbits, Rats, and Mice

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Studies of Heredity in Rabbits, Rats, and Mice

Studies of Heredity in Rabbits, Rats, and Mice

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Excerpt

The foregoing observations show unmistakably that the several members of this allelomorphic series tend, as a result of crosses, to become more like each other. This has been described as mutual modification, but it should be expressly stated that in the light of our experiments with rats "modification" need not be regarded as change in the nature of a single gene, but merely as equalization of the residual heredity additional to the single genes which produce monohybrid ratios.

ENGLISH.

In November 1909 there were received at the Bussey Institution four "English" rabbits, 1 male and 3 females, bred by R. W. Wills, of Hornerstown, New Jersey. In terms of the grading scale shown in plate 3, the male was of grade 2 1/4; the females were of grades 2, 2 3/4, and 3, respectively.

In matings of the male with each of the 3 females, there were produced both English and self-colored young, as shown in table 30; of the former, 21; of the latter, 8. The self young were later found to produce no English young when bred inter se. Hence it seems clear that English is a dominant Mendelian character, that self is recessive in relation to it, and that the 4 English parents were all heterozygous dominants.

The question now arose whether homozygous English rabbits could be produced and why English rabbits were not regularly bred in homozygous form. We did not have long to wait for an answer to these questions. Table 30 shows that the English young of our 4 original English rabbits fall into two groups quite different in appearance. Of the 15 young which were graded, 5 were of grade 1 or 1 1/4, while 10 were similar to the parents in grade, varying from grade 2 to 3. The group of low-grade English was found to consist of homozygous individuals which produced only English young in crosses with each other or with selfs. The higher-grade group, twice as numerous in individuals, was found to consist of heterozygotes. These are preferred by the fancier because of their much more striking colorpattern. The homozygote is in appearance only an impure white animal, but the heterozygote is beautifully mottled. It is therefore clear why the fancier breeds heterozygotes. (See plate 3.)

Our original English buck, 2545, was also mated with self-colored does of several different sorts, viz, gray, cream, yellow, sooty yellow (tortoise), black, and black-and-tan. In regard to color inheritance, these matings gave us such results as are already familiar through earlier publications by Punnett, Hurst, and ourselves. We may therefore confine our attention to the behavior of the English pattern in crosses. Table 30, B, shows the results obtained; 26 English and 18 self young were recorded from these matings. No grade was recorded for 17 of the English young. The others varied in grade . . .

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