Hair shows incredible diversity in curl and texture. While this diversity can be produced naturally, genetics plays a major role. Photo credit: Atiya Walker via Unsplash
Hair is a characteristic trait of all mammals with varied textures, structures, and colours. At the molecular level, hair is composed of a protein called alpha-keratin, which exists in two forms: Type I and Type II. The former is composed of acidic amino acids (the building blocks of proteins), whilst the latter is composed of basic amino acids.
One helix of each type intertwines due to physical forces between them, forming coiled, rope-like strands. Several strands assemble into larger complexes, known as protofilaments, which then combine to form thicker structures called intermediate filaments. Multiple intermediate filaments collectively form the hair cortex, which is then surrounded by the cuticle, an outer structure made from overlapping cells.
Uncovering the structure of hair provides a useful scientific basis for understanding an important facet of our identities, and thus celebrating our differences.
Despite similarities in structure, there is a huge amount of variety in colour, texture, volume, and curl pattern across the population. While this can be influenced by styling, this diversity is ingrained in our genetics. Uncovering the structure of hair provides a useful scientific basis for understanding an important facet of our identities, and thus celebrating our differences.
The Science of Hairdressing
Cuticle cells give hair its great strength and stability. This is owed to an extensive network of disulphide bonds, which form between two sulphur atoms on adjacent parts of the keratin protein. These bonds are amongst the strongest chemical bonds that occur in nature and they hold the key to changing the structure of hair—for instance via waving, curling, or straightening.
The disulphide linkages between keratin hair filaments can be chemically broken using heat. This also disrupts the network of hydrogen bonding, which is important for maintaining the overall structure of the hair cortex. An uncoiling and reshaping effect will, therefore, occur if the hair is bent into a new shape using tongs or straighteners. The application of chemicals then promotes the formation of disulphide bonds in new positions, leading to the hair being waved or curled.
The structure of the proteins in our hair confers great overall strength to individual filaments. This might be an evolutionary explanation for why these proteins are also used to build other biological structures, including skin and wool.
A Genetic Component?
Factors including the shape of the hair follicle and the density of disulphide bonds between keratin filaments are known to influence hair shape, and these are subject to genetic control. One gene thought to be responsible is the gene for trichohyalin, a protein expressed in the inner root sheath cells of hair follicles. This protein contains many charged helical regions that enable it to associate electrostatically with the intermediate filaments of the hair cortex, conferring mechanical strength.
The degree of association and the prevalence of trichohyalin affects the structure of the intermediate filaments, creating changes to the overall texture of the hair. Variations of this gene, called polymorphisms, are therefore thought to be associated with differences in hair morphology, emphasising the role of disulphide bonds in whether hair is curly, wavy, or straight.
Another gene that may also be involved in influencing hair shape encodes the copper transporter protein: CUTC. Recent studies have indicated that controlling copper uptake might be crucial for hair curl formation, with copper deficiencies being associated with discrepancies in wool quality in lambs.
There remain many questions surrounding the full extent of genes involved in hair morphology…
Several genetic disorders also shed light on the biological basis of hair shape. For example, ‘woolly hair syndrome’ results in characteristically fine curls, whereas sufferers of ‘uncombable hair syndrome’ have very wiry and difficult to comb hair. Studying the structural changes caused by these genetic differences will lead to greater understanding of their roles in controlling hair shape.
There remain many questions surrounding the full extent of genes involved in hair morphology, how this varies between different populations, and the science behind styling. It is likely that we are still far from fully understanding the complete range of genetic and environmental factors controlling curls.
