While many believe a single “hair gene” determines everything, hair characteristics are not governed by one gene alone. Instead, they are shaped by a complex interplay of many genes working together. This genetic blueprint sets the stage for the specific hair features an individual develops.
The Genetic Blueprint of Hair
Hair characteristics are determined by an individual’s genetic makeup. These traits are polygenic, influenced by multiple genes. Genes provide instructions for producing various proteins, such as melanin for color and keratin for structure, which are fundamental to hair development.
These instructions are encoded within DNA, organized into chromosomes. Humans inherit 23 pairs of chromosomes, one set from each parent. Specific segments of these genes, known as alleles, carry variations that contribute to the unique expression of hair traits.
How Hair Traits Are Passed Down
Hair characteristics are inherited from both parents. Each parent contributes one allele for every gene, and these pairs of alleles determine an individual’s specific hair traits.
Some hair traits follow dominant or recessive inheritance patterns, where a dominant allele can mask the effect of a recessive one. However, many polygenic hair characteristics involve incomplete dominance, meaning neither allele fully dominates, leading to a blended or intermediate trait. This interaction contributes to the wide spectrum of human hair types.
Common Inherited Hair Characteristics
Hair color is primarily determined by the type and amount of melanin pigments produced: eumelanin for black and brown shades, and pheomelanin for red and yellow tones. The MC1R gene influences hair color, particularly red hair. Variations in MC1R can reduce the production of eumelanin, leading to higher levels of pheomelanin and resulting in red or blonde hair.
Hair texture, whether straight, wavy, or curly, is also significantly influenced by genetics. Genes like EDAR, FGFR2, TCHH, Hr, and KRT2 play roles in determining hair follicle shape and the protein structure of hair strands. Curly hair is often considered a dominant trait, but the interplay of multiple genes means that two parents with curly hair can sometimes have a child with straight or wavy hair.
Androgenetic alopecia, commonly known as pattern baldness, is another inherited hair characteristic that affects both men and women. This condition is polygenic and influenced by androgen hormones. While the AR gene on the X chromosome is a significant contributor, meaning it can be inherited from the mother, numerous other genes on both X and autosomal chromosomes also play a role in its development and progression.