Evidence for a major gene influencing performance on a vocabulary test

Scores on a vocabulary test given to members of 1818 nuclear families of different ethnic backgrounds were subjected to admixture analysis, segregation analysis utilizing both the mixed model and the transmission probability model and linkage analysis with 16 polymorphic markers. The data fitted a commingled distribution better than a unimodal distribution. Tests of the hypothesis that a major gene was segregating were not rejected by mixed model segregation analysis or by transmission probability analysis when provision was made for contribution from factors other than a major gene. There was no detectable heterogeneity by ethnic group nor by mating type. These analyses provide consistent support that there is a major gene for a component of verbal ability measured by a vocabulary test, and the segregation pattern is consistent with the expected Mendelian ratios. When vocabulary scores were adjusted for covariates including years of education, academic achievement, and reading habits, evidence for a major locus was lacking. The results of the linkage analysis were inconclusive. Current segregation models are affected by various factors leading to false inferences regarding monogenic mechanisms; however, in many respects, the mixed model and the transmission probability model are complementary with respect to power and robustness. The results of the analysis are discussed in this context.We thank Dr. J. M. Lalouel for access to his modified POINTER program, Professor N. E. Morton for access to NUCLEAR and POINTER, and Professor R. C. Elston for his support. The results reported here are made possible by collaboration of a group of investigators (G. C. Ashton, R. C. Johnson, M. P. Mi, and M. N. Rashad at the University of Hawaii and J. C. DeFries, G. E. McClearn, S. G. Vandenberg, and J. R. Wilson at the University of Colorado), supported by NSF Grant GB34720 and National Institute of Child Health and Human Development Grant HD06669.