The curliness of human hair is primarily influenced by the shape of the hair follicle and the distribution of certain proteins in the hair shaft, particularly keratin. The genetic determinants of hair texture are complex and polygenic, meaning that multiple genes contribute to the phenotype.
One of the major genes associated with hair curliness is the trichohyalin (TCHH) gene, which plays a role in the structural integrity of hair. Variants (alleles) of certain genes in the KRT (keratin) family, particularly keratin 75 (KRT75), have also been implicated in determining hair texture.
In summary, a combination of alleles from multiple genes influences whether hair is straight, wavy, or curly, with specific mutations in the genes associated with keratin production being significant contributors to curliness. The interplay of these alleles leads to the variety of hair textures observed in humans.