compounded by the expectation that most of the alleles influencing traits common in the population are common and of relatively small effect (i.e., account for only small quantities of the total trait variance). Such alleles are unlikely to have major effects on the genes in which they lie and thus are unlikely to be obviously functional (unlike the rare mutations that account for rare, strongly genetic diseases like cystic fibrosis or Huntington's disease). Thus, the absence of obvious functional significance does not mean that a variant can be assumed NOT to impact the trait. Knowing whether an associated variant is causal takes considerable additional testing, and often claims of functionality are controversial and contradictory.
