The sex ratios of probands and of secondary cases in conditions of multifactorial inheritance where liability varies with sex.

Some pathological conditions affect one sex more often than the other. A curious feature of some of these conditions (dyslexia, congenital dislocation of the hip, pyloric stenosis, otosclerosis, lupus erythematosus, ankylosing spondylitis) is that the sex ratio (proportion of males) of randomly ascertained probands is more extreme than that of their affected relatives. In all these conditions, multifactorial inheritance has been suspected. A simple model is offered here of multifactorial inheritance with liability varying by sex. Under the model, such a disparity between sex ratios would be expected.