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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112 sex determination some on X becomes uniformly heteropycnotic in mice and behaves as an integral part of X according to an extension of Lyon's hypothesis. Experiments are now in progress to investigate Lyon's theory especially in man. It has been shown in the previous chapter that the number of Barr's bodies corresponds in abnormal caryotypes to the total number of X chromo¬ somes minus one. On the other hand XO, XX, XXX and XXXX individuals have not the same phenotype as they should have if all the X were inactivated but one. Bentler et al. (1962) reported that women heterozygous for a certain enzyme deficiency have two populations of erythrocytes which appear to have either a normal or a deficient enzyme level. It appears at present that, although Lyon's hypothesis has been wholly confirmed in mice, it is lacking complete evidence in man, where portions of X chromosome remain probably still active, as suggested by Russell (1963). The well-established difference between the Drosophila and the mammahan X, which behaves in two entirely different ways seems, however, to be the most promising contribution from the point of view of the action of sex chromosomes in mammals. Researches by Douin (1909), which were later confirmed by Lorbeer (1927), demonstrated that in Sphaerocarpus male and female determining factors are segregated in one of the meiotic divisions since in each spore tetrad, which is surrounded by a common membrane, two of the spores result to produce male gametophytes and two female gametophytes. Researches by Allen (1917) demonstrated the existence in Sphaerocarpus donneili of a big X chromosome in the female gametophyte and of a much smaller Y chromosome in the male gametophyte. They are both present in the sporophyte and again segregate during the spore formation. The great differences in size between X and Y chromosomes are often correlated with the considerable size differences existing between female and male plants in several species of Musei (Fig. 5.8) (Lorbeer, 1927). The Haplophase Sex in Sphaerocarpus Fig. 5.8. A male and a female plant of Sphaerocarpus texanus (Allen, 1919),