MC1R variant alleles show multiplicative effects on red hair penetrance. MC1R effects on skin reflectance are additive. Four common MC1R alleles strongly associate with red hair and fair skin. Three MC1R alleles show low penetrance for red hair. Strong R alleles yield OR 63.3 for red hair. Weak r alleles yield OR 5.1 for red hair. Red-haired individuals are mostly R/R (67%) or R/r (11%). Eye color modifies MC1R effects on skin color, freckling, and mole count. OCA2 SNPs associate with brown/green eyes, not blue. R/R individuals with brown eyes have twice the mole count of those with blue eyes. Other OCA2 polymorphisms likely influence mole count. Study involved 2331 adolescents from 645 twin families.