Heredity in ophthalmology / Jules François.

  • François, J. (Jules)
Date:
1961
Catalogue details

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

Credit: Heredity in ophthalmology / Jules François. Source: Wellcome Collection.

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    16 GENERAL GENETICS Fig. 1. Schematic representation of the chromosomes in a cell in the process of division. (After Kemp, 1929.) cut in two lengthwise (Fig. 2). Half of each chromosome passes into each of the daughter cells so that the latter contain again 46 chromosomes, forming an exact replica of the chromosomes in the mother cell. This is called somatic division or mitosis. Before the next division takes place, the chromosomes have resumed their original thickness; then the same longitudinal division takes place again so that the granddaughter cells also contain the same 46 chromosomes. Thus, somatic cells generally contain these 46 chromosomes arranged in 23 pairs or in 2 haploid sets and are said to be diploid cells (ôtTrXoOf, double). When the gametes (the ovum and spermatozoon) are being produced, a special type of cell division occurs and is called reduction or miotic division (fxeLwcTLs, reduction). There is no longitudinal division of the chromosomes. The 23 pairs dissociate, each chromosome of one pair separating itself from the other and passing alone, but whole, to the daughter cell (Fig. 3). It is not the chromo¬ some which divides but the pair, with the result that one ovum or one spermato¬ zoon carries only a single element of each pair, i.e., 23 elements making up 1 haploid set of chromosomes. It is for this reason that the gametes are said to be haploid cells (àirXovs, simple). At fertilization, each chromosome of the spermatozoon meets its corre¬ sponding homologous chromosome in the ovum and, combining with it, recon¬ stitutes a pair. In this way, the fertilized egg will again contain 46 chromosomes, of which 23 come from the mother and 23 from the father. The 23 pairs thus reconstituted will remain the property of the cells of the new individual right up to the moment when this individual will, in turn, form his own gametes with the haploid set of 23 chromosomes. The chromosomal division peculiar to the gametes explains how the chromo¬ some number, and consequently the hereditary material, is not doubled with each cell division but remains constant. It explains also how the two parents contribute equally to the development of the hereditary characters of the child