Human hair varies in color, texture, and thickness. How these traits are passed down through generations involves a complex interplay of genetic factors.
The Basics of Hair Genetics
Hair characteristics are encoded within genes, segments of DNA that carry instructions passed from parents to offspring. Humans inherit two copies of each gene, one from each parent.
Genes have different versions, called alleles. While some alleles are dominant or recessive, many hair traits are determined by multiple genes acting together, a phenomenon called polygenic inheritance.
How Genes from Both Parents Shape Your Hair
Hair traits are generally influenced by genes inherited from both the mother and the father. This is rarely a simple “either/or” situation. Instead, it is a complex interaction involving many genes from both parental sides. The polygenic nature of hair inheritance means that numerous genes work in concert to determine characteristics like color, texture, and growth patterns.
Because multiple genes contribute to hair traits, predicting a child’s hair characteristics based solely on one parent’s hair is often inaccurate. Each parent contributes a unique set of alleles, and the combination of these alleles from both parents shapes the child’s hair. This complex genetic blending explains the wide variation in hair types observed within families and across populations.
Inheritance Patterns of Specific Hair Traits
Hair color is determined by the type and amount of melanin pigments produced in the hair follicles. Eumelanin creates black and brown hues, while pheomelanin results in red and yellow tones. The MC1R gene is well-studied for its role in hair color, particularly red hair, but many other genes also influence the production and distribution of these pigments, leading to a spectrum of colors.
Hair texture, whether straight, wavy, or curly, is also influenced by multiple genes. Genes like EDAR and FGFR2 are associated with hair thickness and texture in Asian populations, while TCHH has been linked to texture in people of Northern European ancestry. The shape of the hair follicle, which is genetically determined, influences the curliness or straightness of the hair strand. Hair thickness, referring to the diameter of individual strands, is also a polygenic trait.
Male pattern baldness, or androgenetic alopecia, has a strong genetic component. While often believed to be inherited solely from the mother’s side due to the androgen receptor (AR) gene located on the X chromosome, this is a simplification. Research indicates that male pattern baldness is polygenic, involving multiple genes inherited from both the mother and the father. Although the X-linked AR gene contributes significantly, other genes from both parents also play a role in the likelihood and pattern of hair loss. Studies on twins suggest that genetics account for approximately 80% of male pattern baldness.
Beyond Genes: Other Influences on Hair
Beyond genetics, various non-genetic factors can modify hair appearance and health. Environmental elements, such as exposure to sunlight and air pollution, can impact hair. UV radiation from the sun can damage hair proteins, leading to dryness, brittleness, and color changes. Air pollutants can accumulate on the scalp and hair, contributing to issues like dryness, breakage, and thinning.
Nutritional status also plays a role in hair health. A balanced diet rich in proteins, vitamins, and minerals is important for hair growth and strength. Deficiencies can lead to hair shedding or stunted growth. Hormonal fluctuations, influenced by factors like puberty, pregnancy, menopause, or conditions such as thyroid disorders, can significantly affect hair growth cycles, density, and texture. Stress can also disrupt the hair growth cycle and contribute to hair shedding.