Genetics, Evolution and Man

Abstract

This book would be an excellent text for the genetic and evolutionary section of a general human biology degree. Introductory chapters on cells, chromosomes, Mendelian law, DNA, chromosomal anomalies and gene mutation, somatic cell genetics, and immune mechanisms are clear and comprehensive. The section on population genetics and demography manages to convey principles effectively with the use of only simple mathematics. Darwinian fitness is defined, probably correctly, as intragroup only, and intergroup selection, perhaps at least as important in human evolution, is given only a paragraph. Medical genetics is introduced with a section on deleterious mutations. The section on balanced polymorphism describes the sickle-cell story in some detail and a useful graph is given showing the approach to equilibrium at different levels of heterozygote advantage. Complex types of polymorphism are illustrated by the blood groups and the HLA system. The principles of founder affect and drift are well set out. It would have been useful if the authors had discussed the degree to which some examples of localised high frequency of recessive disorders, for example PKU in Ireland or cystic fibrosis right across Europe, can or cannot be attributed to such random changes and at which level of frequency and popu-lation size it is necessary to invoke heterozygote advantage. The section on molecular evolution is admirably clear and interesting. In another edition it would be useful to have the parallel evidence from cytogenetics. The basic models of polygenic inheritance and the estimation of hereditability are well described, but might perhaps have been extended to include discussion on the possibility of measuring genotype-environmental interaction and how to manage genotype-environmental covariance, for example in relation to IQ. In estimating dominance from a comparison of sib-sib and parent-child correlations the authors should perhaps have warned that a more plausible explanation of a relatively higher value for the sib-sib correlation than dominance is a greater degree of common environment. In suggesting that stability over the growing period is required by a largely genetic determination of IQ the authors perhaps have not appreciated that change in IQ with growth in childhood may itself be genetically deter-mined. Comparing monogenic and polygenic threshold models for common diseases the authors prefer the former, on the grounds that 'they correspond to situations that are simpler in physiological terms'. The reviewer would prefer the model which is likely to come closer to the true situation in physiological terms. In this section a table on the percentage of relatives affected with some common malformations is misconceived and needs correction in the next edition: the figures ascribed to first cousins (third degree relatives) are those which would apply to second degree relatives, and those for second cousins are those that would apply to first cousins. No study of common malformations is yet available which gives figures for second cousins and it would be a formidable task to collect such data. In the final section on evolution, human welfare and society the authors include a concise and up-to-date account of the fossil record of human evolution, some applications of genetics to medicine, and some of the social implications of human genetics. The authors estimate the date of the separation of the human line from the Australopithecine at some 3 million years ago. They place the origin of modern man at about 40 000 years ago, and consider it likely that this type of man spread from a single centre, probably in the Near East over most of the rest of the world by some 30 000 years ago, largely replacing but perhaps with some intermixture with local popu-lations of man. An elegant map illustrates the spread of the neolithic civilisation, this time certainly starting from the Near East, about 10 000 years ago and moving at about 1 kilometre a year. Again it is a matter of conjecture to what extent this was a spread of populations or just of culture. In the section on genetics and medicine frequency of recessive and X-linked conditions is correctly given but that of dominant conditions is a serious underestimate. This is probably because the authors' estimate is based on the Vancouver and Birmingham surveys of children, which largely omitted the dominant diseases with onset in adult life, such as monogenic hypocholesterolaemia, adult polycystic disease of the kidney, and Huntington's chorea, which are naturally the common ones. The section on social 297