Clinical genetics / edited by Arnold Sorsby.

Date:
1953
Catalogue details

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

Credit: Clinical genetics / edited by Arnold Sorsby. Source: Wellcome Collection.

    29/626 (page 10)
    penetrance and expression undiseased transmitters, however, has invariably revealed that they possess one or another of the haematological abnormalities, particularly hyperfragility of the erythrocytes, which characteristically are associated with the frank disease (Race, 1942). Consequently, so far as currently available data go, we may regard the gene as fully penetrant in heterozygotes (if we are considering any and all effects of its presence which are detectable) but as variable in expressivity ; alternatively we might refer to the gene as completely penetrant with respect to certain of its haematological effects, but incompletely penetrant in respect to its expression as frank disease. T 2 à e ¿ ¿ А i à <^'<|> i Al ái¿¿ □ Normal male О о Normal female Normal sibs of both sexes d.i. Died in infancy Fig. 3.—Pedigree of Thomasen, E. (1948). a Э C€ Ее \ Cataract Myotonic dystrophy, no cataract Myotonic dystrophy, cataract not stated Propositus' myotonic dystrophy. {After Myotonia. Opera ex Domo Biologlae hereditariae húmame Univ. Hafniensis, 17.) The extent to which an evaluation of penetrance, unless qualified, depends on the acuity of examination may be further illustrated by reference to Fig. 3 which shows a fairly representative pedigree of myotonic dystrophy (Thomasen, 1948). Cataract is frequently found, and when it is not obvious, slit-lamp examination usually (though not always) reveals lenticular opacities of a rather characteristic type. Of the 47 patients examined by Thomasen with the slit-lamp, 6 had manifest cataract, and the characteristic slit-lamp opacities were seen in 35 more. In the pedigree shown here, there are 5 individuals without signs of myotonic dystrophy among whose progeny the disease nevertheless appears ; 3 of the 5 (1, 2 and 3 on the chart) exhibited manifest cataract, while in numbers 4 and 5 (aged 62 and 46 years respectively) cataractous changes in the lens were discovered only on slit-lamp examination. From the numerous pedigrees of myotonic dystrophy recorded by Thomasen and by others, it is difficult to escape the conclusion that an autosomal gene is involved which is manifested as myotonia and dystrophy (with or without cataract) in about 60 per cent of the heterozygotes in whom it is present, and as cataract,, without signs of neuromuscular disorder, in 20-40 per cent more. Irregular dominance Genes with effects which are sometimes, but not invariably, manifested in heterozygotes are often referred to as irregularly dominant, which is another way 10