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.

    33/312 (page 13)
    SEX-LINKED INHERITANCE Dominant X chromosomal inheritance The theoretical possibihties in the case of a dominant gene situated on the non¬ homologous part of the X chromosome are illustrated in Figs. 6 a and b. If the father is affected (Fig. 6 a) none of his sons can be affected, for all his sons get their X chromosome from their mother (the father contributing the Y chromosome). All the daughters are bound to be affected, as all daughters would have an X chromosome contributed by the father carrying an abnormal gene. (This is another variety of criss-cross inheritance, the sex of affected children being opposite that of the affected parent.) If the dominant gene is carried by the mother (Fig. 6 Ъ) sons and daughters would both fall into two types; one half would have a normal X chromosome, the other half an abnormal X chromosome; 50 per cent of both the sons and the daughters would, therefore, be affected. The ratio of affected to unaffected of each sex is 1 : 1, and this does not differ from the distribution observed in an autosomal dominant affection. Y chromosomal inheritance Just as the greater part of the X chromosome has no counterpart in the Y, so there is a portion of the Y chromosome {c-d in Fig. 5) which has no counterpart on the X chromosome. The genes it carries are few in number and consequently pathological conditions are very few. When such are present, they must be ex¬ pressed and the affection will occur in men only, and will be transmitted from generation to generation, all the sons being affected. As there is nothing to balance the action of a gene in the c-d segment of the Y chromosome, the question of dominance or recessiveness does not arise. Partial sex-linkage It will be recalled that there is a portion of the Y chromosome (b-c) which has a counterpart in the X chromosome {see Fig. 5). The inheritance of a pathogenic gene situated on these homologous portions of the sex chromosomes is in some ways no different from that observed with two autosomal chromosomes, and for many years conditions now recognized as possibly due to partial sex-linkage were regarded as autosomal. The possible theoretical situations are as follows : (1) The pathogenic gene is carried by the mother only, either in the simplex or the duplex state (Fig. 7 a and Ъ). (2) A pathogenic gene is carried by the father only, either in the simplex state, when it may be on the X chromosome or the Y chromosome, or in the duplex state (Fig. 1 c, d and e). (3) The pathogenic gene is carried by both parents. Here again the mother may be either heterozygous or homozygous; the father may carry the gene on either the X or the Y chromosome, or in the duplex state (Fig. 7 f-k). Of the many possibilities shown in Fig. 7 only a few are likely to occur clinically. Recessive genes If only one gene is pathogenic and is recessive, it will obviously not manifest itself. In offspring of carriers the sex ratio will vary according to whether the gene is carried by the father on the homologous parts of the X or the Y chromosome (Fig. 7 / and g). 13