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.

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    2 SEX DETERMINATION meaning—is generally shown by specialized stages of the reproductive cycles where genetic recombination has been demonstrated. The problems of sex determination are concerned with the factors of the widespread—and maybe essential—phenomenon of sex differentiation and incidentally with some aspects of incompatibility in reproduction. It can be said that, from a general point of view, sex determination is obtained through the segregation of differences at meiosis. The mechanisms that lead to this result are extremely varied and may come into action at different stages of the sexual cycle. It is important therefore to make a quick survey of the basic chromosomal and cytoplasmic processes that make possible both the unbroken trans¬ mission of heredity and the determination of individual and gametic sex diversity in the course of the reproductive cycle. Reproduction and Chromosomes The first decisive step to the modern understanding of reproductive pro¬ cesses was accomplished when it was reahzed that, since organisms are all composed of cells (Schleiden, Schwann), reproduction is necessarily linked with cellular division. The study of mitosis or karyokinesis (Schneider, Strassburger) rapidly lead to the discovery of the chromosomes (Waldeyer, 1890) and of their exact distribution in the nuclei of the daughter cells after mitosis. 1. Mitosis and its Significance in Heredity The sequence of mitotic processes is now sufficiently known in its morpho¬ logical aspects and it appears to take place in basically the same way in the cell divisions of all organisms, except bacteria. The most impressive event occurring at the beginning of mitosis is the resolution within the nucleus of separate bodies which were called chromo¬ somes because they were discovered, and are still generally examined, by employing peculiar basic stains. They were carefully studied from a morpho¬ logical point of view as soon as their importance as bearers of hereditary factors was realized. Three main types of chromosomes have thus been described: telocentric or acrocentric, subtelocentric and metacentric. In the first type the chromosome is rodlike and the centromere is located very near to one of the ends, the subtelocentric chromosome has two unequal arms with the centromere on one side and the metacentric chromosome has two equal arms. Secondary constrictions, points of attachment of the nucleoli, regional differences in staining properties, and other minor peculiarities in form, behaviour and structure proved to be constant in individual chromo¬ somes. The constancy of their basic number in each species is assured by the regularity of the mitotic process.
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