Sex determination.

  • Guido Bacci
Date:
[1965]
Catalogue details

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

Credit: Sex determination. Source: Wellcome Collection.

    87/328 page 71
    GONOCHORISM AND SEX DIGAMETY 71 twelve autosomes and two X chromosomes, the males one X chromosome and a smaller Y hétérochromosome. The two X chromosomes of the females pair at meiosis, migrate to opposite poles of the achromatic spindle and in this way all the female gametes receive an X chromosome. In the male sex, after a partial pairing of X and Y, each of them goes to a different pole so that one half of the gametes will receive six autosomes and the X chromosome (6 A + X) and the other half receives the same haploid set of autosomes and the Y chromosome (6 A + Y). Egg fertilization with the X bearing sperm will develop a female zygote (12 A + XX) and fertilization with the other sperm type will give rise to a male zygote (12 A + XY). The homogametic and the digametic sexes have the same number of chromosomes in the Lygaeus type and only the shape and the structure of the Y chromosome are different from those of the X chromosomes. This makes the two categories of male gametes cytologically identifiable. The fruit fly, Drosophila melanogaster, belongs to the Lygaeus type. The female has three pairs of autosomes and one pair of rod-like X chromosomes. In the male there is only the rod-like X chromosome, the other member of the pair being represented by a Y chromosome with a hook shaped or bent end. Females have thus the chromosome constitution 6 A + XX and males 6 A + XY. It has been ascertained that in man normal males possess both X and Y chromosomes and therefore they produce X or Y bearing sperm the same as a number of nematode, insect and mammalian species, which have visible X and Y chromosomes. The first example of sex chromosome in plants has been discovered in the liverwort Sphaerocarpus where a very large X chromosome and a much smaller Y chromosome have been discovered (Allen, 1917). A similar situation exists among Angiospermae in Melandrium, first studied by Winge and by Blackburn (1923), where the Y is bigger than the X chromosome. In some species differences between the X and the Y chromosomes are not detectable by differences in shape or size but the XY pair may stiU be recog¬ nized by its behaviour as in the case of the hemipteran Oncopeltus fasciatus where the X and Y chromosomes precede autosomes during the anaphase migration (Wilson, 1909-12). Finally in many species both of plants and animals the sexual digamety of the male sex can only be demonstrated by genetical methods since the X and Y chromosomes cannot be identified cytologically. 2. The Protenor Type of Male Digamety The first researches by McClung on grasshoppers referred to a situation where the female homozygous sex has two X chromosomes and the male sex one X and no Y chromosome. In Protenor belfragei, Wilson demonstrated that the female has fourteen
    digitised image 1
    digitised image 2
    digitised image 3