Complex segregation analysis of two principal components derived from horizontal and vertical head size traits

Background: There is a wealth of publications establishing the involvement of genetic factors in the determination of inter-individual variability of head size traits. However, little is known about the mode of inheritance of craniofacial traits in the healthy population.

Aim: The aims of this study were to investigate the mode of inheritance of horizontal (HOC) and vertical (VEC) components of head dimensions, and to test the hypothesis of a common major gene for these traits.

Subjects and methods: The study was conducted on 1406 individuals belonging to 357 pedigrees. Univariate and bivariate complex segregation analyses were conducted on two principal components, HOC and VEC, extracted from 10 original head traits.

Results: The hypothesis of Mendelian transmission was accepted in both studied traits. The inferred major genes explained 54.0% and 45.6% of HOC and VEC variance, adjusted for covariates. For both traits an additive mode of major gene alleles interaction was suggested. No positive evidence for a common major gene for both HOC and VEC was obtained.

Conclusion: We conclude that head size in horizontal and vertical dimensions is determined by two different major genes together with modest and minor effect genes, the latter being partly shared by HOC and VEC.