Animal Breeding Abstracts 12: 151 (1944)

Fisher, R. A., and Holt, S. B. 1944 [Univ. Coll., London.] The experimental modification of dominance in Danforth's short-tailed mutant mice. Ann. Eugen. [Camb.], 12: 102-120.
An experiment is described in which Sd mice from Dunn's stock were outcrossed, and positive and negative selection lines set up for longer and shorter tails resp. In the positive line tail length increased in the successive matings, tails up to 5/6 of normal length occurring frequently in the offspring of later matings. The av. gain in each generation was 2.32 mm. In the negative line tail length diminshed rapidly at first but after the second selection little or no progress was made. There is evidence of adverse natural selection. Mortality of heterozygotes during the first 3 wks. following birth was systematically higher than that of normal homozygotes from the same matings, except in the positive line where viability of heterozygotes improved. There was no apparent overlap in the frequency distributions of the 2 lines. Rapid selective change has had the unexpected effect of shortening the av. generation length. The av. tail length of normal homozygotes in both lines has been unaffected. A viable short-tailed homozygous (female) occurred in the positive line. In the presence of favourable modifying factors homozygotes could undoubtedly live to maturity and be capable of breeding. Selected material from an outbred extension to the main experiment was introduced into the positive line after a period of 2 yrs. in order to maintain an adequate supply of modificatory variance without any immediate setback to its progress.


Collected papers of R. A. Fisher, p. 491 (1974)
Rapid selective change has had the unexpected effect of shortening the average length of the generation. Comparisons are made between (a) the interval from mating of parents to birth of mice selected as parents, and (b) the interval from mating of parents to birth of young in general.