This paper reexamines classic attempts at estimating the number of possible genotypes for a species. In the original computations (Hirsch, 1963), the probability that any two human parents will produce two offspring with the same genotype was calculated to be equal to (1/223)2, or over one chance in 70 trillion. The error lies in that this number reflects the number of cells in the matrix of zygotes, not the number of zygotic genotypes. When this is taken into account, the probability of two human parents producing two offspring with the same zygotic genotype is 1 in 160,000—almost a billion times more likely than previously suspected. The complexity of the genetic system is also discussed in the context of the concept of “heritability,” often confused with that of “heredity.” This confusion has led to the wrong view that heritability represents a nature/nurture ratio.