Genetics of human body size and shape: complex segregation analysis

The genetic component in a mixed heritability model, including major gene, multifactorial and sibling environment transmissible components, was studied for some 20 anthropometric traits in two ethnically different samples. The first sample comprised 305 Mexican nuclear families, and the second comprised 83 Ashkenazi Jewish nuclear families living in Israel. All variables, after adjustment for age and sex, were subjected to principal-components analysis (PCA) in each sample separately. The mixed model of inheritance as implemented in the computer program POINTER was used in the present study. An attempt was made to evaluate genetic effects in the variation of the first principal components (PCs). PCA showed a clear separation of variables and was easily interpretable. The PC1 was a credibly general size factor. This factor alone accounted for about 30% of the total variance. Other components are rather shape factors for various combinations of traits. The testing of several genetic hypothesis showed the following: (1) For all factor scores the genetic component was high and statistically significant. (2) In a Mexican sample, for PC1 and PC2, both major gene and polygenes contributed significantly to the total variation of these variables; in the major locus the alleles were most probably codominant. (3) For all other PCs the major gene effect hypothesis was rejected, with more than 50% of the variation attributable to the polygenes contribution. (4) No evidence was found for sibling environmental resemblance in either sample.