Studies on inbreeding. III. The effects of inbreeding, with selection, on the sex ratio of the albino rat

Abstract

ONE FIGUREDuring the latter part of the nineteenth century it was generally believed that sex in man and in various animals is determined mainly by the amount of nourishment that the embryos receive; well nourished embryos were supposed to become females; those that were poorly nourished were assumed to develop into males.A considerable amount of evidence in favor of this view was collected by Diising ('83, '84, '86), who maintained, furthermore, that close inbreeding interferes with embryonic nutrition, by lessening the vitality of the mother, and so produces a great excess of male young.In the literature of the succeeding twenty years that deals with the subject of sex determination, Dusing's statement regarding the effect of inbreeding on the sex ratio was widely quoted and generally credited. Those who challenged the truth of the assertion were, in the main, advocates of the ancient theory, generally ascribed to Hippocrates (460-377 B.C.), that sex is determined in the ovary; eggs from the right ovary producing males and those from the left ovary developing into females. During this period three series of experiments were made that give data regarding the sex-proportions in a closely inbred stock.Huth ('87) inbred rabbits, brother and sister, for six generations and found a relatively low sex ratio (78.8 3 : 100 0 ) among the ninety young in which the sex was ascertained; Copeman and Parsons ('04) obtained a similar result in their inbreeding experiments with mice. Schultze ('03) concluded that inbreeding DEAN KING has no pronounced tendency to produce an excess of male young, although he found a high sex ratio (110.93: 100 0 ) among 135 mice that were the offspring of brother and sister matings. The question as to whether inbreeding does or does not alter the sex ratio was not satisfactorily answered by any of these experiments, for in each case the number of animals used was small, and there was, apparently, no selection of the best stockfor breeding or any way of checking the results. Moreover, none of these investigations were continued long enough to give evidence that could be considered as conclusive.The effects of inbreeding on the sex ratio seemed to me to be a problem of sufficient importance to warrant a careful and prolonged investigation. For if it were possible to swing the sex ratio of any animal in a definite direction by factors that could be controlled, one might hope to gain valuable information regarding the nature of sex-a problem that has been a favorite subject of speculation for many centuries and one that modern methods of research have not, as yet, satisfactorily solved. MATERIAL, METHOD, AND SCOPE OF THE INVESTIGATIONThe albino rat (Mus norvegicus albinus) was the animal used in this investigation, which was begun in 1909. Details regarding the manner in which the experiments were conducted were given in the first paper of this series (King, '18), but it has seemed advisable to repeat them here in order to give a clear understanding of the way in which the problem has been appro...

The sex ratio and multiple births in cattle1)

“…If the ova have a higher affinity towards Y‐bearing than towards the X‐bearing spermatozoa one would expect a primary sex ratio higher than equality. The hypothesis that such differences in attraction occur in mammals is used by King (1918) to explain the difference in secondary sex ratio between two inbred strains of rats. It is also assumed that the penetration of the ovum may be easier for one type of spermatozoa than for the other (Parkes 1926a).…”

“…King (1918) explains the statistically quite significant difference in sex ratio between two strains of rats, produced by inbreeding and selection, as being due to genetic factors that „act on the metabolism of the ova in such a way as to render the ova more easily fertilized by one kind of spermatozoa than by the other”. According to this hypothesis the mother would be wholly responsible for the degree of deviation from equality of the primary sex ratio.…”

Determinants of Sex Proportions in Man, with Consideration of the Evidence Concerning a Contribution from X-linked Mutations to Intrauterine Death

“…and Paternal Influences The findings of King (1918) had suggested that when inbred lines of rats were crossed the sex proportions of the progeny were mainly determined by the dam. However, Gates (1930), in a linkage experiment in mice in which 8 males sired 1437 offspring, found inconclusive but suggestive evidence that there were different sex proportions in the progeny of some males irrespective of the dam.…”

The lack of effect of ejaculate sequence on sex ratio in rabbits

“…The cause of this disparity is unknown (King, 1918). Dusing (1884) reported an increase in the proportion of female offspring from stallions that were mated frequently, in contrast to the proportion of offspring from stallions mated less frequently.…”