Hair traits, such as color, texture, and thickness, are not inherited exclusively from one side of the family. The idea that hair comes only from your mother or father is an oversimplification of complex biology. Hair characteristics are determined by a combination of genes inherited from both parents, blending the genetic contributions of both family lines. This blend, along with non-genetic factors, results in your unique hair phenotype.
Understanding Polygenic Inheritance
Most hair characteristics are considered polygenic traits, meaning they are influenced by the interaction of multiple genes. This contrasts with simple Mendelian traits, which are controlled by a single gene with straightforward dominant or recessive patterns. Because dozens of genes work together to determine a trait like hair color, the inheritance pattern is much more complex and less predictable than a single gene model.
Every person receives half of their genetic material from each biological parent, including a full set of chromosomes from each side. Therefore, the genes that code for hair traits are always a mix of those passed down from the maternal and paternal lines. The final appearance of the hair results from how all these gene variants interact and express themselves. Both parents contribute equally to the underlying genetic code.
How Hair Color and Texture Are Inherited
Hair color is largely controlled by the type and amount of melanin pigment produced in the hair follicles. The primary regulatory gene is MC1R, which acts like a switch, directing cells to produce either eumelanin (black and brown shades) or pheomelanin (red and yellow shades). A highly functional MC1R gene usually results in darker hair, while variants that reduce its function lead to lighter or red hair.
MC1R is not the only gene involved; several other genes, such as TYR, TYRP1, and SLC24A5, influence melanin production and distribution. The final shade is a spectrum determined by the cumulative effect of all these genes inherited from both parents.
Similarly, hair texture (straight, wavy, or curly) is a polygenic trait influenced by genes like TCHH (Trichohyalin), EDAR, and KRT74. These genes affect the shape of the hair follicle: a round follicle produces straight hair, and an oval-shaped follicle produces curly hair. The specific variants of these texture genes inherited from both family lines combine to determine the degree of curl or straightness. For instance, a child might inherit variants for straight hair from one parent and variants for curly hair from the other, resulting in wavy hair. The expression of these traits is a blended outcome.
The Genetics of Hair Loss
The most common form of hereditary hair loss, Androgenetic Alopecia (pattern baldness), is often mistakenly believed to be inherited solely from the mother’s side. This myth originated because one of the most influential genes involved, the Androgen Receptor (AR) gene, is located on the X chromosome. Since men inherit their X chromosome exclusively from their mother, the maternal line was once thought to be the only source of the trait.
While the AR gene is a significant factor in hair follicle sensitivity to dihydrotestosterone (DHT), the condition is polygenic. Numerous other genes on non-sex chromosomes, known as autosomes, contribute to the timing and severity of hair loss. These autosomal genes are inherited from both the mother and the father.
Scientific studies have identified over 250 genetic regions associated with pattern baldness, with the vast majority found on autosomes. The father’s genetic contribution, particularly from these autosomal genes, can significantly influence the overall risk and progression of hair loss. Therefore, while the X chromosome plays a major role, especially in men, hair loss risk is a combination of genetic factors from both parents.
Non-Genetic Factors Influencing Hair Traits
While genetics establish the predisposition for hair traits, non-genetic factors can alter the hair’s appearance and health over a person’s lifetime. Internal biological changes, such as hormonal fluctuations, can temporarily or permanently change hair texture and thickness. For example, the surge and subsequent drop in estrogen during pregnancy and the postpartum period can cause temporary hair shedding known as Telogen Effluvium.
Nutritional status plays a role, as deficiencies in iron, protein, or certain vitamins like Vitamin D can impair the hair growth cycle and lead to thinning. External factors, including psychological or physical stress, can trigger temporary hair loss. Environmental damage from excessive sun exposure, heat styling, or harsh chemical treatments can physically alter the structure of the hair shaft, changing its texture and increasing brittleness.