Quantitative trait loci

The genetic aetiologies of mental retardation can be subdivided into inherited causes, chromosomal abnormalities, single-gene defects, and multiple-gene defects (for accounts of quantitative and molecular genetics see Cha.pter2.4:1 and Chapter2..4.2 respectively). The polygenic threshold (multiple-gene defects) model proposes that in some people, particularly those with mild mental retardation in whom no obvious cause could be detected, there are abnormal loci situated in multiple genes. The accumulating effect of these loci lowers the threshold and increases the vulnerability of these people to develop mental retardation under environmental stress. Whereas in the past, the polygenic-threshold model primarily remained a hypothesis, and vulnerability or liability deemed to have been an unobserved variable, attempts have been made in the recent years to devise quantitative measures of liability under the concept of 'quantitative trait loci'. (2) It is assumed that the liability to develop mental retardation is continuously distributed in the population. It is those whose liability at some point exceeds a certain threshold that manifest mental retardation while under environmental stress.

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