Hair texture, whether straight, wavy, or curly, is a genetic trait passed down through generations. The distinct shape and form of an individual’s hair are determined by genetic information inherited from their parents.
Understanding Genetic Inheritance
Genes are fundamental units of heredity, carrying instructions that determine various traits, including hair characteristics. These instructions come in different versions called alleles. For each gene, an individual inherits two alleles, one from each parent. Alleles can be either dominant or recessive, influencing how a trait is expressed.
A dominant trait will manifest if an individual inherits at least one copy of the dominant allele. In contrast, a recessive trait will only appear if an individual inherits two copies of the recessive allele, one from each parent. The combination of alleles an individual possesses for a specific gene is known as their genotype. The observable characteristic that results from this genetic makeup, such as hair texture, is called the phenotype.
The Primary Gene for Hair Curl
Curly hair is a dominant genetic trait, while straight hair is recessive. If a person inherits even one allele for curly hair, they are likely to have curly hair. A significant gene involved in determining hair curl is the trichohyalin gene (TCHH). It produces a protein, trichohyalin, which strengthens the hair shaft.
Variations within the TCHH gene are associated with differences in hair curliness. For instance, inheriting one copy of the dominant allele (represented as ‘C’) along with a recessive allele for straight hair (‘c’) typically results in curly or wavy hair (genotype Cc). Individuals with two dominant alleles (CC) also have curly hair, while those with two recessive alleles (cc) have straight hair.
Beyond Simple Dominance
Hair texture is not always a simple dominant/recessive trait determined by a single gene. While a primary gene like TCHH plays a role, hair texture is often influenced by multiple genes, a phenomenon known as polygenic inheritance. Various genes interact to determine the degree of curl, hair thickness, and overall texture, leading to a wide spectrum of hair types.
Other genes, such as EDAR and FGFR2, have also been linked to hair texture. Environmental factors can subtly influence hair appearance, though DNA is the primary determinant. This complex interplay of multiple genes makes hair texture more intricate than a basic Mendelian inheritance pattern.
Predicting Hair Types in Families
Understanding the genetic basis of hair texture helps predict the likelihood of certain hair types in offspring. Since curly hair is dominant, if one parent has curly hair and the other has straight hair, their children might inherit curly or wavy hair depending on the curly-haired parent’s specific genetic makeup. For example, if a curly-haired parent carries both a dominant and a recessive allele (Cc), they can pass on either allele.
Two curly-haired parents can have a child with straight hair if both carry the recessive allele for straight hair. In this case, their child could inherit two recessive alleles (cc), resulting in straight hair. The polygenic nature of hair texture means variability can occur within families, leading to a range of hair types, including wavy hair.