Hair texture is a common point of curiosity regarding physical traits. The question of whether straight or curly hair is dominant often arises, reflecting interest in how traits are passed down. Understanding hair type genetics reveals a complex interplay of inherited factors. This article explores the genetic mechanisms behind hair texture, moving beyond simple dominant and recessive ideas.
What Dominance Means in Genetics
Genes are fundamental units of heredity, carrying instructions for various characteristics. Each gene exists in different versions, called alleles. When an individual inherits two different alleles for a particular gene, one allele may mask the effect of the other.
The allele that expresses its trait and masks the other is referred to as dominant. Conversely, the allele whose trait is hidden or not expressed when a dominant allele is present is called recessive. For a recessive trait to appear, an individual must inherit two copies of the recessive allele, one from each parent. For example, in some cases of eye color, a brown eye allele can be dominant over a blue eye allele, meaning a person with one brown and one blue allele will likely have brown eyes.
How Hair Type is Inherited
Hair type inheritance is not as simple as a single dominant or recessive gene, making the question of whether straight or curly hair is dominant more intricate. Instead, hair texture is considered a polygenic trait, meaning multiple genes interact to determine the final characteristic. Researchers have identified several genes that play a role in influencing hair texture.
One such gene is TCHH (Trichohyalin), which is involved in the structural proteins that make up the hair shaft. Variations in the TCHH gene have been linked to differences in hair straightness or curliness. Another gene, EDAR (Ectodysplasin A Receptor), is associated with hair thickness and straightness, particularly in individuals of East Asian descent, where a specific allele of EDAR contributes to straighter, thicker hair. The FGFR2 (Fibroblast Growth Factor Receptor 2) gene also contributes to hair texture, with certain variants influencing the degree of curl.
The combined effects of these and other as-yet-unidentified genes dictate whether hair will be straight, wavy, or curly. The overall genetic make-up determines the specific shape and structure of the hair follicle, which in turn dictates the hair’s texture.
The Spectrum of Hair Textures
The polygenic nature of hair inheritance explains why hair texture exists along a broad spectrum, moving beyond simple categories like “straight” or “curly.” This variability encompasses not only straight hair (Type 1) and various degrees of curly hair (Type 3 and 4) but also wavy hair (Type 2). Wavy hair, for instance, represents an intermediate texture.
Different degrees of curliness are also observed, ranging from loose waves to tight coils. This wide array of textures arises from the specific combination and expression levels of the multiple genes involved. The shape of the hair follicle, which is genetically determined, significantly influences the hair’s cross-sectional shape and how it grows out of the scalp, directly impacting its texture.