For most complex traits, the genes that have been identified to date account for only a relatively small fraction of the total variation in the trait. One extreme example is adult height. An enormous amount of data is available for height, not because height has been studied extensively for its own sake, but because in studies of common diseases the height of each individual is recorded, and hence data on height are available without additional effort or expense.
375
In one analysis, results from 79 separate studies were combined. These studies included more than 250,000 individuals of European ancestry who were genotyped for common nucleotide variants (SNPs) at about 3 million nucleotide sites. The analysis identified 697 genes affecting height. Some of these genes are known to affect skeletal development, growth hormones, or other growth factors, but most have no obvious connection to the biology of growth. Some of the genes had previously been identified by studies of rare mutations that have pronounced effects on skeletal growth, either in human families or in laboratory mice. An unexpected finding was that a few genes affecting height are known to be associated with bone mineral density, obesity, and rheumatoid arthritis. These and many other of the genes identified might affect height indirectly.
The 697 genes for height identified among the 250,000 individuals account for about 60% of the genetic variation in height, a number that suggests that many more genes with still smaller effect also contribute to variation in height. And this analysis does not address at all the effects of the environment on human height, nor genotype-