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 . 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 .