Weinberg observed that proportions of homozygotes in familial studies of classic 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, by chance no albino child occurs. He reasoned that many carrier couples were not being counted, and he demonstrated methods for correcting results to produce the expected Mendelian ratios. 