Human chromosomes / E.H.R. Ford.

  • Ford, E. H. R. (Edward Hugh Rawlinson)
Date:
1973
Catalogue details

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

Credit: Human chromosomes / E.H.R. Ford. Source: Wellcome Collection.

    69/408 (page 49)
    V. VARIATIONS IN KARYOTYPE IN PHENOTYPICALLY NORMAL PERSONS A number of variations in the human karyotype have been reported in persons who are phenotypically normal. These fall into one of three categories: (a) Variations in chromosome number (aneuploidy). (b) Variations in the structure of individual chromosomes which do not appear to have any genetic effects, and therefore do not probably result from structural rearrangements. (c) Variations in the structure of individual chromosomes which are due to structural rearrangements of their contents and which may therefore lead to abnormal meiosis and possibly to the production of abnormal gametes. A. Aneuploidy in Normal Persons It has been shown (Jacobs et al., 1961, 1963) that the number of aneuploid cells found in blood cultures from normal people increases with age. The change normally consists of loss of a single chromosome. In women the process starts in the sixth decade, and eventually up to 7% of cells may have only 45 chromosomes. The missing chromosome is one of the С group, and since experience shows that cells which are monosomic for an autosome do not usually survive it seems almost certain that the missing chromosome is an X, and probably the inactive member of the pair. In men there is a comparable phenomenon, but it is less striking. There is an increase of aneuploid cells which lack a single small acrocentric chromosome, almost certainly the Y. The effect does not appear until the seventh decade, some ten years later than in women, and the number of aneuploid cells is not usually above 1-2%. There were one or two reports, soon after karyotyping became possible, of differences of chromosome number in different races, but these failed to receive confirmation and may be discounted. B. Variations in Chromosome Structure Without Genetic Effects 1. Variation in the Y Chromosome Y chromosomes which are unusually long (longer than the chromosomes of the F group, and sometimes as long as those of group D), unusually short (less than half the length of the