Comparative genetic variance and heritability of head and facial traits in northwest Indian and Belgian twins



Genetic variance analysis of 13 head and facial traits is considered in samples of Northwest, Indian and Belgian twins. Modified t tests, based on the nested structure of twin data, indicate differences between monozygotic (MZ) and dizygotic (DZ) mean values of bigonial diameter in Indian females and in Belgian males, i.e., for only two of 52 instances. Heterogeneity of variance is observed in about 30% of the craniofacial traits in both samples, invalidating conventional within-pair genetic variance estimates for these traits. Patterns of environmental bias on zygosities differ between sexes within the same population and also between the two samples. Revised average genetic variance ratios are higher in Indian than in Belgian twins. Male twins manifest, on average, higher genetic variance ratios than female twins in both the samples. There is stronger evidence of enviornmental covariance in MZ than in DZ twins for both Indian and Belgian twins. The pattern of intraclass corrlations, based on average values, is as follows: ♂rMZ > ♀rMZ > ♂rDZ ≥ ♀rDZ in the Indian twins and ♂rMZ > ♀rMZ > ♀rDZ > ♂rDZ in the Belgian twins.