Genetics in ophthalmology / Arnold Sorsby.

  • Sorsby, Arnold, 1900-1980.
Date:
1951
Catalogue details

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

Credit: Genetics in ophthalmology / Arnold Sorsby. Source: Wellcome Collection.

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    ILLUSTRATIVE PEDIGREES A pedigree consistent with dominant sex-linked inheritance is shown in Fig. 39. The affected man in the first generation cannot have affected sons, but all his daughters would be affected because the father contributes an X chromosome with a dominant gene. The woman in the second generation transmits the affection to 50 per cent of her children, the sex ratio being the same for sons and daughters. Fig. 39.—Presumed dominant partial sex-linkage: A pedigree of retinitis pig¬ mentosa. On the assumption of dominant partial sex-linkage the affected man in the first generation carried the dominant gene on his X chromosome and can have only affected daughters. Such daughters in turn can contribute their abnormal X chromosomes to either sons or daughters. The man marked A in the third generation would again transmit it to all his daughters and to none of his sons. Actually two of his daughters are unaffected, and one son is affected. This can be explained on three cross-overs occurring in this generation. The affected son В in the fourth generation now carries the pathogenic gene on his Y chromosome and would therefore transmit the affection to his sons only. Actually one of his sons is normal and two of his daughters are affected. This, too, could have occurred as the result of cross-over. {After S. Snell, Trans. Ophthal. Soc. U.K., 1903, 23, 68.) In subsequent generations the same pattern would repeat itself. Affected men transmit the affection to all their daughters ; affected women transmit the affection to 50 per cent of their sons and 50 per cent of their daughters. There are, however, exceptions in this pedigree which might arise from cross-overs. Thus it may be postulated that in the third generation three cross-overs occur in the first sibship of 10 children leading to one affected son and two normal daughters (the crossing occurring at gamete formation in A). The affected son, B, now carries the gene on the Y chromosome and transmits it to his sons only: this is seen by the fact that 6 of his 7 sons are affected. Of the 4 daughters none should be affected; actually 2 are affected here. Three cross-overs explain the anomalous sex distribution. Criss-cross Inheritance This may occur in several distinct modes of sex-linked inheritance. The theoretical possibilities are shown in Table III. Recessive sex-linkage.—An affected woman married to a normal man has affected sons and carrier daughters. The daughters are, therefore, phenotypically like their father {see Fig. 32). 45