Hair characteristics, such as color and texture, are inherited through a complex biological process involving contributions from both parents. This process relies on the intricate interplay of multiple genes, explaining the diversity of hair traits observed across individuals.
The Genetic Blueprint of Hair
Hair traits are determined by genes, which are segments of DNA inherited from parents. Each individual receives two copies of every gene, one from the mother and one from the father. These gene copies, known as alleles, dictate specific characteristics. When these alleles are passed down, they combine in various ways, influencing the visible traits.
Some traits follow a dominant-recessive pattern, where a dominant allele can mask the effect of a recessive allele. For instance, if one parent contributes a dominant allele for a certain hair trait and the other a recessive allele, the dominant trait will be expressed. Hair inheritance is often more intricate, involving multiple genes that interact to produce a wide spectrum of outcomes.
How Specific Hair Traits Are Inherited
Hair color is influenced by several genes that control the type and amount of melanin produced. Melanin is the pigment responsible for hair color, primarily existing in two forms: eumelanin, which dictates black and brown shades, and pheomelanin, which contributes to red and yellow hues. The gene MC1R is a significant factor in determining red hair, with specific variants leading to its expression. Red hair typically results when an individual inherits two copies of a recessive variant of the MC1R gene, one from each parent.
Brown hair is generally considered a dominant trait, meaning only one copy of the dominant gene is sufficient for its expression. Conversely, blonde hair is recessive, requiring two copies of specific recessive genes to manifest. Genes like HERC2 and OCA2 also interact to influence hair color, alongside eye and skin pigmentation. The interaction among these various alleles, rather than a single gene, ultimately determines an individual’s hair color.
Hair texture, whether straight, wavy, or curly, is genetically determined and is considered a polygenic trait, meaning multiple genes contribute to it. The TCHH gene (Trichohyalin) is associated with hair texture, particularly straight hair in individuals of European ancestry. Another gene, EDAR, influences hair thickness and straightness, especially in East Asian populations. The shape of hair follicles, which is regulated by genes like Hr, directly affects the curliness or straightness of hair. Hair texture is an example of incomplete dominance, where a combination of alleles can result in an intermediate trait, such as wavy hair from curly and straight hair genes.
The Role of Multiple Genes and Other Influences
Hair loss, particularly androgenetic alopecia (pattern baldness), is a complex inherited condition. While a common misconception suggests it is solely inherited from the mother’s side through the X chromosome, genes from both parents contribute to its likelihood. The androgen receptor (AR) gene, located on the X chromosome, is linked to male pattern baldness, explaining the maternal influence. However, other genes inherited from the father’s side also play a role in the onset and severity of hair loss. Beyond genetics, hormonal factors and environmental influences can affect hair traits and hair loss over time.