Variants of the melanocortin 1 receptor (MC1R) gene are common in individuals 
with red hair and fair skin, but the relative contribution to these pigmentary 
traits in heterozygotes, homozygotes and compound heterozygotes for variants at 
this locus from the multiple alleles present in Caucasian populations is 
unclear. We have investigated 174 individuals from 11 large kindreds with a 
preponderance of red hair and an additional 99 unrelated redheads, for MC1R 
variants and have confirmed that red hair is usually inherited as a recessive 
characteristic with the R151C, R160W, D294H, R142H, 86insA and 537insC alleles 
at this locus. The V60L variant, which is common in the population may act as a 
partially penetrant recessive allele. These individuals plus 167 randomly 
ascertained Caucasians demonstrate that heterozygotes for two alleles, R151C and 
537insC, have a significantly elevated risk of red hair. The shade of red hair 
frequently differs in heterozygotes from that in homozygotes/compound 
heterozygotes and there is also evidence for a heterozygote effect on beard hair 
colour, skin type and freckling. The data provide evidence for a dosage effect 
of MC1R variants on hair as well as skin colour.