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.

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    BIOMETRIC EVALUATION OF FINDINGS example, on good grounds it may be assumed that only in those who are infected with syphihs can general paresis (dementia paralytica) develop, which might be due to an inherited disposition. The latter character is consequently constel- lational and due to syphilis infection and a coincident factor which probably is genetic and brings out a peculiar reaction that gives rise to general paresis. However, if the incidence of syphilis in a population were 100 per cent, the existence of general paresis would Ьг wholly dependent on the hypothetical genetic factors. Any particular character is actually due to both genetic and environmental factors. Without genetic factors there would, of course, be no individuals. Nor can an individual live in an excessively abnormal environment which, for instance, is too hot or too cold. Within the hmits set by death genetic and environmental factors may combine in many different ways. The classification suggested here is an attempt to create some order in this muddle. It should be noted, furthermore, that we are used to think of characters as normal or pathological. What we mean by normal is not quite clear. Actually we mean different things in different situations. The normal, generally speaking, is equivalent to the usual. From the genetic point of view normal characters are said to be complexly inherited, whereas the pathological as a rule seems to depend on a simple genetic factor. Naturally, though this process is the usual one it is not necessarily the only one, and it must not be assumed that pathological char¬ acters never can be complexly inherited. This is well illustrated by the simple fact that there is no sharp and definitive frontier between normal and pathological. Besides, smaU deviations from the normal are usually called anomalies, provided that they are not accompanied by any marked functional disorder. INHERITANCE IN THE PEDIGREE The older medical literature, but also a few recent works, often specified the existence of inheritance in the pedigree of a certain trait. The implication of the term is that the character occurs in the person's relatives, chiefly his ancestors. Such data are not particularly useful by themselves. The situation is better and more satisfactory if figures are given for a control group with the character lacking in the initial persons. A considerable difference would then carry some force of proof. It should be kept in mind, however, that persons with a given disease are particularly interested in finding the same disease in members of their family or amongst their relatives. Merely for this reason they should get higher figures, that might suggest the existence of inheritance in the pedigree, than the controls without the character. A difference may naturally be due also to random variation, especially if there are few persons in the group. It should finally be noted that some genetic characters appear at an earlier age than others. Consequently the groups to be compared must have fairly similar age distributions. It is also necessary in both groups to study persons of the same kind of relationship. It will seem that these facts are seldom appreciated. The worst is that often it is impossible to tell whether they have been. If a character has the frequency p in the population and if data are available for an average of 10 relatives, it is easy to calculate how often the character should 84