weinberg observed that proportions of homozygotes in familial studies of classic autosomal recessive genetic diseases generally exceed the expected mendelian ratio of 1:4, and he explained how this is the result of ascertainment bias. in his work with albino children, he recognized that in some families where both parents carry a recessive mutation, no disease occurs by chance. he reasoned that many carrier couples were not being counted, and he demonstrated methods for correcting results to produce the expected mendelian ratios.