In the post Is there a gene for alcoholism? I explained how claims to discover the gene for such and such complex behavior (mostly alcoholism and homosexuality) are based on correlations that are never confirmed by experimentation. We will have to wait until neurogenetics comes of age before we can seriously tackle this kind of question. But when that happens, how likely is it that we really discover a gene for alcoholism?

To make my point come across, I will have to touch a few words about the problem of missing heritability. According to current estimates, the human genome consists of ~ 25,000 protein-coding genes and about as many non protein-coding RNAs, the function of which still remains to be established. The implicit meaning of "gene for alcoholism" is actually a mutation that would somehow affect one of these ~ 50,000 functional entities.

Mutation is somewhat inaccurate in this context as we should speak of polymorphism. A piece of our genome is monomorphic if everybody has exactly the same sequence, otherwise, it is polymorphic. The vast majority of polymorphic sequences in humans are SNPs (single-nucleotide polymorphisms), i.e. sequences that differ by only one nucleotide...