Eye colour: HERC2 and OCA2
HERC2 regulatory variants affecting OCA2 explain a substantial share of iris-colour variation in people of European ancestry. Intermediate colours, other populations and gene interactions undermine any ‘one SNP—one colour’ rule.
Red hair and freckles: MC1R
Several MC1R variants substantially increase the probability of red hair, fair skin and freckles, often with two functional alleles. Incomplete penetrance and other genes mean the genotype does not draw an exact portrait.
Earwax and odour: ABCC11
rs17822931 is an unusual example of a variant with a large effect on dry/wet earwax and axillary odour precursors. Microbiome, hygiene and perception remain outside this simple marker.
Lactose: MCM6 regulates LCT
Enhancer variants in MCM6 maintain adult lactase activity, but different populations evolved different variants. European rs4988235 is not a universal global test, and symptoms also depend on dose, gut health and microbiota.
Bitterness: TAS2R38
PAV/AVI haplotypes distinguish sensitivity to test compounds PTC/PROP reasonably well. Translation into liking broccoli, coffee or a diet is much weaker: experience and culture can outweigh the receptor effect.
Alcohol flushing: ALDH2 and ADH1B
Reduced-function ALDH2 rs671 is strongly linked to acetaldehyde accumulation and flushing; ADH1B rs1229984 alters the rate at which acetaldehyde is produced. These variants are best studied in East Asian populations. This is not an ‘easy intoxication gene’: acetaldehyde exposure is harmful, and absence of these variants does not make alcohol safe.
Height and chronotype: thousands of small contributions
A 5.4-million-person height study identified over 12,000 associations. A large chronotype GWAS found hundreds of loci. This is biologically informative but does not turn one ‘height gene’ or ‘night-owl gene’ into a personal forecast; environment, age and ancestry still matter.
Sport and personality: do not select a person by SNP
ACTN3 and ACE show small, heterogeneous group associations that cannot choose a sport. Modern personality GWAS find many variants with minuscule individual effects: this rejects the old one ‘adventure gene’ idea rather than creating a DNA personality test.