Genetics for the clinician / C.A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1964
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

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

    72/438 (page 42)
    42 GENETICS FOR THE CLINICIAN children, we count the family either twice or three times as the case may be. Our forty-eight albinos are therefore counted as coming from forty- eight famihes (though in fact they only derive from thirty-seven, i.e. 27 + 9 + I, see Table 3.1, column 2). For ease of demonstration the assump¬ tion is made that each family consists of three children. The Table (3. i) therefore is as follows: Table 3. i It will be seen how important it is that the propositus is left out in each family. If he were left in the ratio would be i : i, seventy-two normal to seventy-two affected. As it is there are 72 normal to 24 affected, i.e. 3:1. The reason that the propositi are left out is in order to compensate for all those sibships which are also the result of the mating of two heterozygotes but in which no albinos appeared. In the example given the ratio of normal to affected is i : i. The difficulty would be in testing for significance if the ratio were not obvious. (3) Expected Genetic Ratios in Sex-linked Inheritance It can easily be seen whether a gene is totally Y-linked because, if it is, the trait wiU only be handed on from father to son and never be trans¬ mitted through a woman; a likely example in Man is hairy ears, (Dro- namraju, i960). With total X-linkage the matter is not quite so straight¬ forward but with a httle thought the expected ratios can be worked out. Thus with a sex-linked recessive on the X,none of the sons ofan affected man could either show or carry the condition provided the mother was not a carrier. His daughters, however, would аД be carriers. A carrier woman, on the other hand, would produce half affected sons and half carrier daughters, her other children being normal (pedigree 3.4). If the trait is totally X-linked and dominant, again none of the sons of an affected man will inherit the gene, but his daughters will all show the