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.