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.

109/328 (page 93)

INTERSEXUALITY AND BALANCE THEORY OF SEX 93 It is nevertheless very important to know the real distribution of the male and female sex determining factors within the X chromosomes and within the autosomes of Drosophila, that is whether the X chromosome contains a single F locus and whether M determiners are distributed over all autosomes in Drosophila. It is possible that in some species only single F and M loci exist. The mechanism discovered in Drosophila, is nevertheless very important in under¬ standing most cases of sex determination. 5. The Search for Female Sex Factors in the X Chromosome Dobzhansky and Schultz (1931, 1934) added or subtracted segments of the X chromosome to normal diploid males or females of Drosophila or to triploid intersexes in order to investigate its sexual content. The use of suitable gene markers allowed to evaluate the extent of each fragment of X. Most of the experiments were carried on triploid intersexes because the vicinity of their sexual balance to the critical thresholds for maleness and femaleness makes them particularly sensitive to the action of both genetica! and environmental factors (Dobzhansky, 1930). Duplications and deficiencies induced, however, detectable effects in diploid male and female individuals also. It was established that the addition of a fragment of X to 2X3A individuals always shifts intersexuality in the female direction and that the extent of the shift is roughly proportional to the length of the broken fragment. Individuals with deficient X showed on the contrary a shift of intersexuality in the male direction. Long duplications conferred to diploid females the appearance of super- females and the appearance of intersexes to diploid males. Only the short pairing segment proved to be devoid of sexual influence in either direction. Dobzhansky and Schultz concluded from their experiments that the X chromosome in Drosophila does not contain a single F locus but it contains multiple female sex determiners which are located throughout its length with the exception of the pairing segment. Even if male sex determiners are also present in the X chromosome, they are located in the vicinity of stronger female determining factors. Therefore the single sections of X are female determining as the whole X chromosome. The Dobzhansky and Schultz experiments raised the problem whether the multiple factors influencing sexual differentiation must be considered as sex modifiers or as proper sex factors. In other words some perplexity arose in deciding whether the factors located in the fragments of X chromosomes simply modify phenotypic characters whose presence might actually be de¬ termined by another factor (Punnett, 1933). Goldschmidt (1935) put the question in the same way and suspected that regions of X not yet appro-