Human genetics and medicine / by Cyril A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1970
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Human genetics and medicine / by Cyril A. Clarke. Source: Wellcome Collection.

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    §3-1 GENERAL CONSIDERATIONS 15 of predisposing genes, and this is why more of her relatives will also have this high concentration of genes and show the disease. This seems rather comphcated but it serves very clearly to illustrate the threshold effect seen in multifactorial inheritance. The study of quantitative variation is of course statistical and the mathematics that are involved are not for the amateur, but fraser Roberts (^9^3) gives a very simple introduction to the principles, which are based on the resemblance between relatives. For each step farther away in relationship, the number of genes in common with one's relatives is halved. A parent passes on to a child half his or her chromosomes; they have half their genes in common. Clearly, the child will pass on half the genes de¬ rived from that particular parent, so a grandparent and a grandchild will have a quarter of their genes in common, and so on. With a rare* dominant autosomal gene, it will be clear on a little consideration that half the children of a person in whom it appears will receive it, a quarter of his grandchildren and an eighth of his great-grandchildren. Half the parents and sibs of an affected person are therefore like the patient and half are unlike, and the likelihood of the other types of relative being affected can be calculated in a similar way. With multifactorial inheritance, however, many genes are combining together to produce the end result. On the average^ half of the genes are making the sib or parent just like the subject, while half of them are making them no more like him than would be an unrelated subject. The difference is that instead of half the sibs or parents being totally like, and half totally unlike, all the parents and all the sibs are tending to be half-like. All the uncles and aunts are tending to be one-quarter-like; all the cousins one-eighth-like. These measures of resemblance are termed regressions^ and multifactorial inheritance would follow the pattern indicated if there were no dominance and all the genes were intermediate in effect. This, of course, they are not, so dominance and recessiveness have to be considered and this involves complications. One effect of dominance in multifactorial inheritance, however, remains the same even though there are variations in dominance from gene pair to gene pair, and differences in the amount of effect produced by different genes : the reduction of the regression for sibs is always half the reduction for parents (meaning, as will be realized, that sibs are more like the subject than are parents). The matter is a complicated one, but the above remarks will give the reader an idea of the arguments involved. The section on high blood pres¬ sure which follows describes some of the investigations which are carried out when it is desired to determine whether a condition is multifactorially inherited or controlled by a single gene. * If it were not rare, it might come in from both sides of the family and the model would not apply.