Hair texture—whether straight, wavy, or curly—is among the most noticeable human traits. Many people seek a simple genetic explanation for how it is passed down, often asking if straight or curly hair is dominant. Hair texture inheritance is not governed by a single switch. It is controlled by a combination of multiple genes working together. Understanding this trait requires moving past simple models to appreciate the complexity of how genetic instructions translate into physical hair shape.
The Traditional Dominance Model
The inheritance of hair texture was often simplified in introductory biology as a classic example of Mendelian genetics. This traditional model proposed that the allele for curly hair was dominant over the allele for straight hair. Inheriting just one “curly” gene variant would result in curly hair, while straight hair required two “straight” variants, one from each parent. This simple model suggested a clear, all-or-nothing outcome. Modern genetic studies have demonstrated that this binary classification is inaccurate and fails to explain the variety of hair textures observed.
The Spectrum of Hair Texture
The presence of wavy hair complicates the simple dominance model, suggesting that hair texture is better described as a spectrum. Wavy hair is the result of a blended or intermediate phenotype, a pattern of inheritance known as incomplete dominance. This means that neither the “straight” nor the “curly” genetic instruction is entirely dominant. An individual with wavy hair often carries one genetic variant for straight hair and one for curly hair. This blended outcome contradicts the rules of simple dominance, where the dominant trait should mask the recessive one.
Genes That Control Hair Shape
The physical shape of the hair strand determines its texture and is linked to the structure of the hair follicle. Straight hair grows from follicles that are nearly round in cross-section, while curly hair emerges from follicles that are oval or flattened. This follicle shape is controlled by genetic machinery.
Key Genes
One significant gene identified in Europeans is Trichohyalin (TCHH), which accounts for about 6% of the variance in hair texture. TCHH provides instructions for a protein that helps cross-link keratin filaments, impacting how the hair structure forms within the follicle. Additionally, the Ectodysplasin A Receptor gene (EDAR) influences hair characteristics, particularly in East Asian populations, where a specific variant is associated with thicker, straighter hair. These genes code for structural proteins and cellular patterns that physically mold the hair strand.
Why Predicting Hair Texture is Complex
Predicting a child’s hair texture is difficult because the trait is polygenic, meaning it is influenced by the combined action of multiple genes, not just one or two. Each of these genes contributes a small, additive effect to the final outcome. This complex genetic architecture is why the Punnett squares used for single-gene traits fail to accurately model hair inheritance. The final texture is determined by the cumulative effect and interaction of many genetic variants inherited from both parents. This interplay of numerous genetic components reinforces why hair texture inheritance is far from a simple dominant or recessive scenario.