Sex differences in genetic determinants of craniofacial variations--a study based on twin kinships

Race, sex, nutritional status and cultural factors affect craniofacial morphogenesis. Out of these, sex is a major factor in craniofacial differentiation, because it can be stronger in one ethnic group and weaker in another. In this study, sex differences in genetic variance and heritability of 13 craniofacial traits are investigated. The study is based on a sample of 45 MZ and 101 DZ twin pairs and their 125 singleton siblings, 104 fathers and 103 mothers in 146 families drawn from an urban population of Chandigarh. Results of t-tests for equality of the means reveal association of zygosity with the mean value of bigonial diameter in female twins and for none in males. Heterogeneity of variance is observed in about 50% traits in females as compared to 15% in males. This invalidates conventional within-pair genetic variance estimates for these traits. The revised genetic variance ratios are higher on an average in males than in females. However, there is greater MZ environmental covariance in male twins than their female counterparts. Family data indicate higher maternal effect for ear height, nasal height and frontal breadth, while greater paternal effect is seen in cranial traits. Sex-wise midparent-child regression coefficients show greater heritability in daughters for nasal traits and bigonial breadth, while sons show higher genetic component for head size measures.