Genetic basis of hair medulla by twin study.

Data on the incidence of medulla of human head hair based on 9,600 strands examined from 48 pairs of zygosity--determined German twins were quantified as medullary ratio and subjected to variance analysis to give information about genetic significance of the character concerned. The results of F test and heritability estimate yielded significant values indicating the presence of a strong genetic component of variability in the incidence of medulla of human head hair. Sex influence upon the character is found to be absent.