A method for the estimation of genetic correlations based upon analysis of genetic marker phenotypes is presented. At given marker locus, the probability of observing a pair of individuals with a specific combination of phenotypes can be expressed as a function of the gene frequencies at that locus and the genetic correlation (R) between that pair. The likelihood of obtaining a smaple of n such pairs with their phenotypes at m marker loci can be expressed as a product of nm such functions. From the likelihood function, maximum estimates of R can be obtained, and hypotheses about R may be tested. A sample of Swedish twin families (61 dizygotic twin pairs, 268 husband-wife pairs, and 164 sib pairs) were analyzed by this method using information from 21 markers. It was found that for the twin pairs, R=0.458, which was significantly different from the R calculated for sib pairs (R=0.558) but not significantly different from the expected 0.5. For the husband-wife pairs, it was found that R=0.086, which did differ significantly from the expected value of 0, indicating the presence of nonrandom mating in this population.
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