An extrapolation of the twin study technique, family correlation studies rely on the fact that relatives share a proportion of their genes with affected individuals. The closer the relationship of relatives, the more genes will be shared and the greater the expression of a common trait. The degree to which a trait is manifest similarly in relatives is termed the correlation. It is measured from zero - no correlation - to one - complete correlation.
If parents are not blood relatives, then the correlation of a quantifiable trait should be at the same level as that of the general population.
Familial correlations for multifactorial traits such as are in keeping with the number of genes that are shared. For example, for the discontinuous trait of cleft lip, 4% of first degree, 0.6% of second degree and 0.3% of third degree relatives have the trait.
Ref: Essential Medical Genetics, J.M. Connor & M.A. Ferguson-Smith, Blackwell's Publishers, 2nd edition.
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