Hair texture, ranging from straight to tightly coiled, is a noticeable characteristic. This variety in hair patterns is not random but arises from specific biological mechanisms. The differences are rooted in the shape of the structures from which hair grows, the internal composition of the hair strands, and the genetic instructions that orchestrate these features.
The Blueprint: How Follicle Shape Dictates Curl
The primary factor determining whether hair is straight, wavy, or curly is the shape of the hair follicle embedded within the scalp. Hair follicles are tiny, tube-like structures that produce hair strands. The cross-sectional shape of this follicle dictates the shape of the hair strand as it emerges.
A perfectly round hair follicle produces straight hair, allowing the hair strand to grow out with a circular cross-section. In contrast, an oval or elliptical follicle yields wavy or curly hair. The more flattened or asymmetrical the oval, the curlier the hair will be, ranging from loose waves to tight coils. The hair strand essentially takes on the mold of its follicle, much like toothpaste squeezed from a shaped dispenser. The angle at which the follicle tunnels into the scalp also influences the curl, with angled follicles promoting curlier growth.
The Building Blocks: Hair’s Internal Chemistry
Beyond the follicle’s shape, the internal chemical structure of the hair strand itself plays a significant role in its texture. Hair is primarily composed of a fibrous protein called keratin. This protein forms long chains that are held together by various chemical bonds, influencing the hair’s strength and flexibility.
Two types of bonds are particularly relevant to hair texture: disulfide bonds and hydrogen bonds. Disulfide bonds are strong, permanent chemical links formed between sulfur atoms in the amino acid cysteine, which is abundant in keratin. These bonds contribute to the overall stability and shape of the hair, with more numerous and unevenly distributed disulfide bonds leading to curlier hair patterns. Chemical treatments like perms or relaxers work by breaking and reforming these disulfide bonds to alter hair shape.
Hydrogen bonds are weaker, temporary connections between water molecules and hair’s keratin proteins. These bonds cause temporary texture changes, such as straight hair becoming wavy or curly hair frizzier in humid conditions. When hair gets wet, these bonds break, allowing reshaping. As hair dries, new hydrogen bonds form, locking it into its new, temporary configuration until it encounters moisture again.
The Inheritance Factor: Genes and Hair Texture
An individual’s hair follicle shape and chemical composition are determined by genetic makeup. Hair texture is a polygenic trait, influenced by multiple genes rather than a single one. These genetic instructions are passed down from parents to offspring, explaining why hair texture often runs in families.
Scientists have identified several genes that contribute to hair curliness, though the exact interplay is complex. Variations in genes like TCHH (trichohyalin) and EDAR link to hair texture differences across populations. These genes influence hair follicle development and structure, and proteins within the hair shaft. While some genes have a more pronounced effect, the combined action of many genes ultimately determines the inherited curl pattern.