Curly hair, known for its distinctive spirals and waves, is a common human trait. Its formation involves a complex interplay of genetic factors and the intricate anatomy of the hair strand. Understanding these scientific principles reveals why some individuals have straight hair while others exhibit a range of curl patterns.
The Role of Genetics
Hair curliness is primarily determined by an individual’s genetic makeup. It is considered a polygenic trait, meaning multiple genes work together to influence the final hair texture. This complexity means that hair inheritance is not always a simple dominant or recessive pattern.
Several genes contribute to hair curl. For instance, variations in genes like TCHH, which helps stabilize the inner root sheath, and EDAR, influencing hair thickness, are linked to different hair textures. Other genes, such as KRT74 and LPAR6, also play roles in hair bending, coiling, and follicle development. While curly hair is often considered a dominant trait, the precise degree of curliness is influenced by the combination and interaction of these various genes.
Anatomy of a Curly Strand
The physical shape of the hair follicle, located within the skin, is a primary determinant of whether a hair strand will be straight or curly. Hair follicles that are perfectly round typically produce straight hair. In contrast, oval or asymmetrical hair follicles give rise to curly hair, with a flatter oval shape generally leading to tighter curls.
The angle at which the hair follicle emerges from the scalp also contributes to curl formation. Hair follicles that tunnel vertically into the scalp tend to produce straight hair, while those angled into the skin cause the hair to curl as it grows.
The internal structure of the hair shaft itself is asymmetrical in curly hair. Keratin proteins, the primary building blocks of hair, are unevenly distributed within the hair fiber, accumulating more on the concave side of the curl. This uneven distribution contributes to the elliptical cross-sectional shape of curly hair strands. Disulfide bonds, strong chemical links between cysteine amino acids in keratin proteins, also play a significant role. These bonds help maintain the hair’s coiled shape; a higher number of disulfide bonds generally results in curlier hair.
Diversity in Curl Patterns
The spectrum of curl patterns, ranging from subtle waves to tight coils, arises from variations in the factors that create curl. Differences in hair follicle shape, the angle of hair emergence from the scalp, and the specific genetic predispositions inherited by an individual all contribute to this diversity. Hormonal fluctuations throughout life, such as during puberty or pregnancy, can also influence the shape of hair follicles and thus alter curl patterns.
Common curl classification systems, such as the Andre Walker Hair Typing System, categorize hair into types based on its natural texture. Type 2 hair describes wavy patterns, which can range from loose S-shaped waves to more defined ones. Type 3 hair encompasses various curly patterns, characterized by distinct loops or spirals. Type 4 hair refers to coily or kinky textures, featuring very tight curls often in a zigzag pattern. Within each type, subcategories (A, B, C) further differentiate the curl based on its diameter or tightness, with ‘A’ being the widest and ‘C’ the tightest within its group, and it is common for individuals to have more than one curl pattern on their head.