How rare is your combination of traits?

Because the trait distributions in the US and UK differ substantially from global averages. Blue eyes occur in 9% of the global population but 27% of the US/UK population (reflecting European ancestry prevalence). Blonde hair is 2% globally but 16% in the US/UK. If you have blue eyes and blonde hair, you are quite rare globally but much more common in a European reference population. The global mode compares you to all 8.2 billion people; the US/UK mode compares you to a European-ancestry baseline. Both are valid depending on what you want to measure.

Morton’s toe refers to a foot structure where the second toe is visually longer than the big toe. It occurs in approximately 22% of people globally, based on AncestryDNA population genetics data and multiple anatomical studies. Its prevalence is consistent enough across populations to use as a bonus rarity modifier. A note on range: studies have found rates from under 3% (in a strict Swedish measurement study) to 42% in some American university populations, reflecting genuine variation in definition and measurement. The 22% figure is the best available global estimate.

It means that if you selected a random person from the global (or US/UK) population, you would need to check approximately X people before finding one with the same combination of traits as you. A result of "1 in 1,000" means your trait combination is shared by roughly 8.2 million people globally. A result of "1 in 1 million" means roughly 8,200 people currently alive share your exact combination. The "people on Earth" figure converts the fraction directly: 8.2 billion × your probability fraction.

Not entirely, and the calculator acknowledges this. Eye colour and hair colour share pigmentation genetics via the MC1R gene: red hair and green eyes co-occur more frequently than simple probability would predict. Blood type and eye/hair colour are genuinely independent, as they are controlled by entirely separate genetic loci. Handedness is genetically influenced but not strongly linked to pigmentation traits. The independence assumption works reasonably well for most combinations, but the calculator displays a caveat on results involving correlated traits. The "1 in X" figure should be read as a close approximation rather than a precise probability.

AB negative is the rarest of the eight common ABO/Rh blood types, occurring in approximately 0.5 to 1% of the global population (slightly higher in some European populations). It is rare enough to be a significant practical concern in blood banking: AB negative donors are actively recruited because their plasma is universal and their red cells are needed for other AB negative patients. By contrast, O positive is the most common type globally, occurring in approximately 38% of people.

Red hair results from variants in the MC1R gene that cause pheomelanin (red/yellow pigment) to predominate over eumelanin (brown/black pigment). Most MC1R variants are recessive: both parents must carry the variant for it to produce red hair in offspring. The highest global concentrations are in Ireland and Scotland, where up to 13% of the population has red hair, reflecting a founder effect in the Celtic population. Globally, red hair occurs in approximately 1 to 2% of people. The gene variants responsible are ancient, with red hair appearing in early European populations before dispersal diluted the frequency.

Papadatou-Pastou et al. 2020, a meta-analysis of 2.39 million participants, is the most comprehensive study on this question and is the source used for this calculator. It found 10.6% left-handedness using the writing-hand criterion. The figure has been remarkably consistent across cultures and historical periods, suggesting a stable biological component. Left-handedness is slightly more common in men than women. Forced switching from left to right hand was common in 20th century education in some countries, meaning older adults may be underrepresented in self-reported left-handedness data.

Eye colour data is more variable than blood type data, which comes from large clinical databases. Global estimates rely on a combination of genetic population studies and survey data. The global green eye estimate of 2% and the US/UK estimate of 9% come from different sources and measurement approaches: genetic association studies tend to produce different figures than self-report surveys because individuals classify eye colour differently (many "hazel" eyes contain significant green). The figures used in this calculator represent the best available estimates, but true prevalence varies somewhat by how eye colour categories are defined.

Mathematically yes, but it requires the independence assumption to be treated strictly. If you select a very rare blood type (AB negative, 0.5%), rare eye colour (green, 2%), rare hair colour (red, 1.5%), left-handedness (10.6%), and all three bonus traits, the combined probability approaches and can exceed 1 in 8 billion. In practice, because many combinations involve correlated traits, the actual rarity is somewhat less extreme. The calculator caps displayed rarity at 1 in 8 billion to avoid implying a precision the underlying data cannot support.