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.

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    52 GENETICS FOR THE CLINICIAN (4) Racial Differences Primaquine sensitivity, as opposed to favism (see p. 256) is controlled by a single gene, the effects of which are much more marked in Caucasians than in Negroes and presumably in the latter the gene-complex partially inhibits the action of the gene. Many other similar instances will doubtless come to light if race crosses are investigated, and in seaport towns, where they are plentiful, it would be interesting to observe the disease patterns in these hybrids as compared with the parent races. Further, it might be profitable to look at the sex ratio in such hybrids—though accurate in¬ formation is hard to come by—to see if Haldane's rule (glossary) is operative (see also p. 84). (5) Similar Phenotypes Produced by Different Genetic Mechanisms The example in the butterflies (Plate 4.3) should keep us aHve to the fact that a character such as a disease may be produced by different sets of genetic mechanisms. Thus retinitis pigmentosa may show either autosomal dominant or recessive, or sex-linked recessive inheritance. In the last, the female carriers can be recognized by the characteristic fundus (p. 287). Some forms of hereditary ataxia may be dominant and others recessive, while in the dystrophies the facio-scapular-humeral varieties are usually but not always autosomal dominants, and the pseudo-hypertrophic type is generally but not invariably controlled by a sex-linked recessive gene. (6) Varying Manifestations of Gene Effects. Pleiotropy Two examples in Man loosely parallel the situation in butterfly No. 4 on Plate 4.4, though the unmasking of the genes in Man is affected in both cases by an environmental and not a genetic factor. Thus the PTC tasting phenomenon (see pp. 217, 315), known to be present in the great apes, could not have given direct evidence of its presence until the synthesis of PTC in 1932. Secondly, DDT resistance in flies, also genetic, can only have shown itself since the introduction of the insecticide. The answer to the question 'What were these genes doing before PTC and DDT were dis¬ covered?' is quite shortly 'something else'. Genes probably never have one single phenotypic effect and the classical example of multiple actions is the t-locus in the mouse (Dunn et al., 1961). Here are numerous alleles and the first mutant (T) to be identified had a short tail when heterozygous for this and the normal allele (t), but T was lethal in the homozygous state. All the other alleles at the locus (designated t with a superscript—e.g. t^, t^)