It is shown that the well known excess of females in anencephalic births is statistically significant when compared with (a) the general population and (b) their normal sibs.
There is also a statistically significant difference between the sex ratio of anencephalic births and that of spina bifida births which suggests that there is a definite difference in the genetic component of their aetiology.
These results are discussed with reference to the genetics of anencephaly and spina bifida and a genetic model of the inheritance of the genetic component of anencephaly is given. The significance of a possible cytoplasmic factor is also considered.
KeywordsGeneral Population Genetic Model Genetic Component Spina Bifida Definite Difference
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