Genetic studies in mental subnormality.

Date:
1973
Catalogue details

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

Credit: Genetic studies in mental subnormality. Source: Wellcome Collection.

    32/100 (page 18)
    GENETIC STUDIES IN MENTAL SUBNORMALITY 18 tions for the analysis of the present data. It is a well recognized characteristic of traits best explained on a multifactorial hypothesis that the sex ratio of those affected departs significantly from the secondary sex ratio in the population. It is also characteristic that when an index case in a sibship is of the less commonly affected sex, then the frequency in sibs is higher than when the index case is of the sex more often affected. This is intelligible in terms of a higher threshold in the less frequently affected sex. As already noted, it does not seem likely that there could be a simple greater liability of males independent of an underlying genetical pre¬ disposition which would influence the aggrega¬ tion of sexes of those affected in sibships. Nevertheless, an analysis on such an hypothesis is instructive and will now be considered. It is postulated that the excess of sibships with only males affected could be due to males having greater susceptibility than females to being affected. As shown earlier, in the present study the proportion of affected males = = o-66. 271 Therefore in this investigation the chance '/>' of a male being affected would be о-66 and the chance of a female being affected would be о • 34. Then the expected distributions of sibships according to sex and number of affected members would be as {p-^-qY = i. Sibships with only one affected member There were 60 sibships with one affected member. (^2 = I'44 I degree of freedom 0'30>P>0'20.) Sibships with two affected members There were 63 sibships with two affected members. MM MF FF Total Expected No. .. .. .. .. .. 27*44 28-28 7-28 63-0 Observed No. .. .. .. .. ЗЗ'О iQ'O 11-o бз'О (^2 = б-о8 2 degrees of freedom 0*05>P>0'02) Sibships with three affected members There were 23 sibships with three affected members. MMF and ' МММ MFF FFF Total Expected No. .. .. 6-6i i5*49 o-go 23-0 Observed No. .. .. .. .. 4-0 17-0 2-0 23*0 (Numbers too small to have confidence in a significant result at the 5 per cent level.) Sibships with four affected members There were four sibships with four affected members. MMMF, MMMM MMFF, etc. FFFF Total Expected No 0-76 349 0-05 4-0 Observed No. .. .. .. .. 2-0 2-0 — 4-0 (Numbers too small to have confidence in a significant result at the 5 per cent level.)