Hair texture, which ranges from pin-straight to tightly coiled, is a physical trait visibly determined by an individual’s genetic makeup. The way this trait is passed down through generations often leads to the question of whether wavy hair is a direct result of a specific inheritance pattern. Specifically, people wonder if the intermediate appearance of wavy hair is an example of incomplete dominance, a model frequently taught in introductory biology.
Defining Incomplete Dominance
Incomplete dominance describes a form of inheritance where one allele is not fully expressed over its paired allele, resulting in a blended phenotype in the heterozygous individual. The heterozygous state produces a new, third phenotype that falls between the two homozygous possibilities.
This concept is often illustrated using the color of snapdragon flowers. When a plant homozygous for red flowers is crossed with a plant homozygous for white flowers, the resulting offspring are all heterozygous and display a pink color. Neither the red nor the white allele completely masks the other, leading to a visible blending of the two pigment expressions.
The 1:2:1 ratio of genotypes and phenotypes in the next generation is a hallmark of incomplete dominance, differing from the 3:1 phenotypic ratio seen in standard Mendelian complete dominance. In this inheritance pattern, the genotype of an organism can be directly determined by observing its phenotype, since the intermediate trait has a unique genotype.
The Classic Single-Gene Model for Wavy Hair
The idea that wavy hair is an example of incomplete dominance stems from a simplified model used to teach non-Mendelian inheritance. In this model, a single gene controls hair texture with two primary alleles: one for curly hair and one for straight hair.
If we assign the allele for curly hair as C and the allele for straight hair as S, a person inheriting two curly alleles (CC) would have very curly hair. Conversely, an individual with two straight alleles (SS) would have completely straight hair. The key to the incomplete dominance interpretation lies in the heterozygous combination (CS).
In this model, the CS genotype results in wavy hair, which is a physical blend of the curly and straight phenotypes. Wavy hair is neither as tightly coiled as the homozygous curly hair nor as flat as the homozygous straight hair, perfectly fitting the definition of an intermediate trait.
Why Hair Texture Is More Complex Than a Single Gene
While the single-gene model is useful for illustration, the actual inheritance of human hair texture is far more intricate and is best described as polygenic. Polygenic inheritance means that multiple genes interact to contribute to the final appearance of the trait, making the simple incomplete dominance model insufficient. Modern genome-wide association studies have identified at least eight distinct genetic locations that influence the curve and shape of hair.
The physical characteristic of hair texture is fundamentally determined by the shape of the hair follicle from which the hair strand grows. A follicle with a perfectly circular cross-section produces straight hair, while a follicle with an increasingly oval or asymmetrical cross-section produces wavy or curly hair. These different genetic variants influence the structure of the hair follicle and the proteins that build the hair fiber.
Specific genes like TCHH (Trichohyalin) have been strongly associated with hair texture variation in people of Northern European descent. The EDAR gene, which affects hair follicle development and the thickness of the hair strand, is also involved in texture differences across various populations.
Environmental and Hormonal Modifiers
Beyond the complexity of polygenic inheritance, a person’s hair texture is not static and can be significantly altered by factors unrelated to their underlying genetic code. External environmental conditions play a role in the temporary appearance of hair texture. For example, high humidity causes the hair shaft to swell and disrupt protein bonds, often resulting in increased frizz or a change in wave pattern. Conversely, low humidity can lead to a loss of moisture, making the hair feel coarser or less defined.
Internal biological changes, particularly shifts in hormone levels, can cause profound alterations to hair texture throughout a person’s life. Hormonal fluctuations during puberty often initiate a change in hair texture, sometimes causing straight hair to become wavy or vice versa.
Pregnancy involves a significant rise in hormones like estrogen, which can prolong the hair growth phase and temporarily change the hair’s feel, often making it appear thicker or altering the curl pattern. Later life changes, such as menopause, involve a decline in estrogen and progesterone, which can shift the balance toward androgens, leading to hair thinning or changes in curl pattern.