Genetics for the clinician / C.A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1964
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics for the clinician / C.A. Clarke. Source: Wellcome Collection.

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    CHAPTER THREE GENETIC RATIOS One of the first results of an interest in genetics is that the clinician is asked to look at pedigrees and give an opinion as to the future prospects of a family. Even though in practice the great majority of conditions enquired about do not follow any obvious Mendelian pattern, yet it is essential to master the simple ratios before the problem of the inherited component in common diseases can be properly appreciated. It must be emphasized that the ratios to be described are, in Man, almost always confined to rare disorders. (i) Expected Ratios where the Condition is Controlled by an Auto¬ somal Dominant Gene It is evident from a study of the behaviour of the chromosomes (Chapter II) that where one parent is heterozygous for a given dominant trait and the other normal (pedigree 3 . i) the offspring wiH segregate in a i : i ratio, half affected and half normal. This is known as a back-cross, and is the common type of mating where a rare trait is concerned. If the mating is between two heterozygotes, three out of four children will show the condition and only one will be homozygous normal (pedigree 3 .2). When we are investigating what may be either a i : i or a 3 : i ratio in a 'dominant', all we need do is to count those individuals showing and not showing the trait and then carry out significance tests on our results to see whether they, in fact, differ significantly from a i : i or a 3 : i ratio respectively (see Chapter 20, pp. 304-10). hctero -a h(2.tero3ygous - affected Ьогпозудои s -normaL Pedigree 3.1. The back-cross to the recessive. 40