Genetics for the clinician / C.A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1964
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics for the clinician / C.A. Clarke. Source: Wellcome Collection.

    61/438 (page 33)
    THE CHROMOSOMES OF MAN 33 have been particularly interested (Walker and Harris, 1962) consisted of individuals in which a 13-15 translocation had occurred, but there were no known trisomies. The translocation carriers have of course 45 chromo¬ somes instead of 46, as in the mongol carriers, and they are normal (or such abnormalities as are present are not considered to be due to the trans¬ location) and this is because the total chromatin is almost normal. It will be seen from pedigree 2. i that the ratio of translocation carriers to non- carriers is I : I but there is not, as in the mongol families, one trisomie in four children. The reason for the difference between the two types of translocation may be that there is increased lethality in the 13/15 one because here the trisomie would receive a considerable quantity of extra chromatin, since chromosomes 13 and 15 are bigger than 21. Genes on the Y Chromosome Influencing the Rate of Maturation in Man The role of the Y chromosome—how important it is and exactly what it does—^is a matter of great speculation. An interesting investigation into the rate of growth in patients with Klinefelter's and Turner's syndromes has been carried out by Tanner and his colleagues (1959). They found that patients with Klinefelter's syndrome (XXY) matured at the same rate as normal males (XY), and that those with Turner's syndrome (XO) matured at the same rate as normal females (XX). As normally males mature more slowly than females, it follows that the Y chromosome causes retardation of growth in the presence of either one or two X chromosomes, and that retardation does not follow either one or two XXs without the presence of Y. It is, therefore, genes on the Y chromosome which cause the normal sex difference in the rate of development. Furthermore, the fact that XXXXY looks male-like and does not produce feminization suggests that feminizing genes are not carried on the X but only on the autosomes (Fraser et al, 1961). In contrast, masculinizing genes must be carried on the Y (see also the Lyon hypothesis, p. 36). It is of interest to compare the situation in cocks and hens, where the former is the homogametic sex (see glossary for explanation of this point). Here, during the first eight weeks of life the growth rate in the two sexes is similar though after this the males grow faster (Hütt, 1949). The hen, however, possesses only one sex chromosome but no Y to retard growth and it would, therefore, seem that it is the double dose of sex chromosome in the cocks which is accelerating growth.