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.

    62/626 (page 43)
    DIAGNOSIS OF ZYGOSITY very soon suffice for nearly all ordinary samples, if the necessary antisera are conveniently available. Blood groups constitute the purest known examples of the ideal diagnostic criteria, for they are simply determined genetically and as completely free of environmental influence in their manifestation as the genes, themselves. Other characters subject to only small environmental variation are the taste-sense (wherein a simple recessive gene substitution confers the inabihty to taste phenyl thiourea and certain substances related to it) and the secretor property (a simple dominant gene being responsible for the property of secreting blood group antigens in body fluids; this property appears to be closely related to the Lewis blood group system). Although the various blood group systems differ widely in their ease of determination, some of them requiring considerable skill and experience to perform successfully, it is interesting to compare their discriminatory power on the assumption that all the necessary antisera at present known are available. This power is dependent on the local frequencies of the genes involved, as indeed the power of any test of this kind is, and the figures quoted here are based on the gene frequencies given for England by Race and Sanger (1950). The MNS system, if anti-^ is used, is the most effective, leaving an undiscriminated residual of about 44 per cent, and the Rhesus is next with about 46 per cent; the Landsteiner groups leave a residual of about two-thirds undiscriminated, while none of the rest (including both the taste and secretor properties) leaves less than about 75 per cent. If all are used in combination (thatis; ABO; MNS; P; Rh; Lutheran; Kell; Duffy; Kidd; Taster; Secretor) there remains a residual of about 4 per cent undiscriminated. The most effective single system could not leave a residual of less than one-quarter of the population, supposing the most advantageous distribution possible—a multiple allelomorphic series with an indefinitely large number of alleles each of vanishingly small frequency, and possessing no dominance relationships. Fingerprints and other physical factors The use of fingerprints in the diagnosis of zygosity requires some method of assessing their degree of similarity, for they are never precisely identical, as their value in criminal investigation might testify. Stocks (1930) has developed a technique for this purpose, and considers pairs possessing seven or more corre¬ sponding patterns to be monozygous. Of the eyes, their colour and more precisely the iris structure and pigment pattern should correspond in monozygous twins; of the hair, its colour, form, and texture should correspond. Dahlberg has put forward the structure and pattern of ears as a criterion; other suggested criteria are the order of development of the carpals, and indeed the order of ossification of all bones is considered to be more concordant in monozygous pairs ; the structure of capillaries ; the presence or absence of hair on the dorsum of the first and second phalanges of the fingers; electrocardiographic, and electroencephalographic traces. Very many other criteria have been employed or proposed for the diagnosis of zygosity, some of them—and not only the more recondite—upon inadequate evidence of their mode of inheritance and susceptibility to environmental influence. Anthropometric measurement A general physical resemblance, particularly in the features, though unrehable is, as Dahlberg remarks, quite good as a guide. This is also true of such quantitative 43