‘Genes of small effect’ considers unidentified genes whose variants collectively affect some characteristic of interest. Many aspects of living organisms depend not on the action of a few genes, but on the actions of many, each having a small effect on the overall characteristic. This assumption has been used successfully to inform the breeding of agricultural animals and plants. But some of the concepts have also been very controversial when applied to human beings. The heritability—the proportion of the variance in a population attributable to genetic variation—of human height, serious mental illness, and IQ is considered along with results from genome wide association studies.
