Shull, George Harrison (July 1908) A new Mendelian ratio and several types of latency. The American Naturalist, 42 (499). pp. 433-451.
Abstract
The foregoing discussion and conclusions may be summarized thus: In certain bean hybrids, mottled seed-coats depend upon the presence of a mottling allelomorph in a heterozygous condition, the homozygous condition giving unmottled seeds. This peculiar situation results in a tripolyhybrid ratio, 18:18:6:6:16, instead of the usual ratio, 27: 9: 9: 3: 16. Latency is held to mean invisibility, and not inactivity or dormancy, and four types are recognized, according to the different causes of invisibility; still other types may be found. The four types discussed in this paper are: (a) Latency due to separation, in which an allelolmorph when acting alone has no external manifestation and is only rendered patent by combining it with another allelomorph. Such latency gives rise to the ratios 9:3:4, 9:7, 27:9:9:3:16 and 27:9:28, instead of the theoretical, 9:3:3:1 and 27:9:9:9:3:3:3:1. (b ) Latency due to combination, in which two dominant allelomorphs, each giving rise to a peculiar character when acting alone, lose their external manifestation when co-existing in the same zygote. Upon self-fertilization this type of latency gives rise to such ratios as 1:1, 3:3:2, 18:18:6:6:16, etc., and may be found to account for the existence of certain mid-races, and other eases in which a double series of characteristics are presented in nearly equal numbers. (c) Latency due to hypostasis, in which the presence of one allelomorph can not be detected owing to the presence of another allelomorph, the character produced by the latter being unmodified by the activity of the former. This type of latency is exemplified by the black bean which hides the presence of a wholly distinct brown allelomorph, and a dark orange bean which carries invisibly a light yellow allelomorph. This condition gives rise in one series of crosses to the ratio, 12:3:1. Properly the term "dominance" should be limited to the relation between any positive characteristic and its own absence. Whenever one positive character seems to dominate another positive character, the latter is latent by hypostasis in the individual possessing the former.(d) Latency due to fluctuation, a very frequent phenomenon in which characteristics disappear under conditions of poor nutrition, etc. Cultivation under favorable conditions makes such characteristics patent and this fact may account in part for the general impression that cultivation induces variation. Cases of "double adaptation" are examples of this type of latency. Many discrepancies between theoretical and empirical inheritance-ratios are due to latency, and care should be taken to investigate the possible latencies which may be present before declaring that a character is non-Mendelian, because of a discrepant ratio. "Variable potency," "contamination" or "impurity" of the gametes, and "alternating dominance" which have been proposed to account for the appearance of various novelties, or of deviations from expected ratios, eau have no secure standing until the question of latency in the sense of invisibility has been taken into account. A modification of expected ratios may rarely result also from the failure of certain allelomorphs to make vigorous zygotes when joined together in certain combinations.
| Item Type: | Paper |
|---|---|
| Subjects: | evolution |
| CSHL Authors: | |
| Communities: | Station for Experimental Evolution at Cold Spring Harbor |
| Depositing User: | Matt Covey |
| Date: | July 1908 |
| Date Deposited: | 15 Jan 2013 21:52 |
| Last Modified: | 02 Nov 2017 14:07 |
| URI: | https://repository.cshl.edu/id/eprint/26779 |
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