Sex determination.

Bacci, Guido.

Date:: [1965]

Catalogue details

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

Credit: Sex determination. Source: Wellcome Collection.

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252 SEX DETERMINATION actually conceal delicate mechanisms of genie balance, and suggested theo¬ retical possibilities for obtaining two sexes with equal frequency. One pos¬ sibility is the appearance of a dominant gene A for large gametes in a diploid population homozygous for gene a which produces smaller gametes. The spread of the gene A would be checked by the difficulty of fertihzation between relatively static gametes, whose advantage (which consists in producing large zygotes) remains only if a sufficient number of smaller a-carrying gametes is produced by the population. This situation would lead to a numeric equihbrium between large and small gametes and would ultimately result in a true sex differentiation. Also a dissortative mating with a preference for the dominant x-recessive types would lead to similar results. The Kalmus-Smith hypothesis provides a simple interpretation for the origin of sex polarity without having recourse to the myth of the primordial male and female principles. The interpretation is based on the other hand on the current theories of monofactorial sex determination although relative sexuality—which is apparently determined by multiple sex genes—most closely approaches the conditions which have been suggested by the authors. It appears, therefore, that the development of the theory on the basis of the results of polyfactorial sex determination would lead to a closer approach to the real conditions of early evolution of sex differentiation. The selective significance of the sex ratio genes has also been discussed with relation to results on multifactorial sex determination in isopods (Shaw, 1958). The already cited results on Ophryotrocha showed on the other hand that autosomal genes influencing sexual differentiation produce monogenic female or male strains when in homozygous condition and it appears that hereditary patterns of sex differentiation decide whether sex genes wiU func¬ tion as sex differentiators or as sex ratio determiners. Sex ratio factors may belong to quite different types like the X-linked gene of Drosophila pseudoobscura or the infective factor which has been investi¬ gated in several Drosophila species. The study of various possibihties that may be presented by autosomal and X- or Y-hnked sex ratio genes presents new evidence for the assumption that the problems of sex ratio and sex differentiation are susceptible of multiple solutions and have been solved in multiple ways in the course of evolution, 3. The Diversity of Genetic Sex Factors, Sex Chromosomes and Heterochro- matin Attempts to build up a genetic theory of sex determination which could be unified and simple in spite of aU variants, have failed, in the present author's opinion, for two main reasons. It has been mentioned already that previous theories were essentially based on the knowledge of the very restricted field of sex digametic organisms. This regarded sex factors as highly specialized entities and a number of other