What this means

TCHH (trichohyalin) codes for a structural protein in the hair shaft's inner root sheath. A common L790M variant (rs11803731) was identified in a GWAS of Australians of European descent as the strongest known single-SNP contributor to straight-versus-curly hair within Europe. It explains only a small fraction of the variance, though — hair shape is highly polygenic, with major contributions from genes like EDAR, FGFR2, and KRT genes, and the dominant determinant in East Asian populations is different from the one in Europeans.

TCHH makes a protein that helps shape your hair fibre as it grows. One version of this gene is the single biggest reason some Europeans have straight hair and others have curly hair — though "biggest" still means it only explains a small slice of the difference. Hair shape is shaped by many genes (plus age, hormones, and treatment), and the genes that drive hair shape in East Asians are different ones again.

Caveats

• This variant predicts hair shape only within European populations; it tells you little outside Europe.
• Effect size is small — this SNP explains roughly 6% of variation in European hair curl.
• Hair shape is influenced by many genes plus age, hormones, and treatment.
• Hair texture often changes after puberty, pregnancy, or with chemotherapy.

References

• Medland et al. — Common variants in the trichohyalin gene are associated with straight hair in Europeans (AJHG, 2009)
• Eriksson et al. — Web-based, participant-driven studies yield novel genetic associations for common traits (PLOS Genetics, 2010)