How rare is your combination of traits?

Because the US and UK look nothing like a global average for the traits you're entering. Blue eyes occur in 9% of people worldwide but in 27% of US adults in the AAO's 2014 survey, and blonde hair in 2% globally but 16% in the US/UK, both a legacy of European ancestry. So blue eyes and blonde hair make you quite rare against all 8.2 billion people and fairly ordinary against a European-ancestry baseline. Pick the mode that matches the crowd you actually want to be compared with, since both answers are true of a different crowd.

A foot where the second toe looks longer than the big toe. It shows up in approximately 22% of people globally, going by AncestryDNA population genetics data and several anatomical studies, and it's steady enough across populations to work as a bonus modifier. Treat the figure loosely. Studies have found anything from under 3% (a strict Swedish measurement study) to 42% in some American university populations, and the spread comes from how each study defined and measured the toe. We use 22% as the best global estimate available.

That you'd need to check approximately X random people from the global (or US/UK) population before finding one who matches your combination of traits. "1 in 1,000" means roughly 8.2 million people alive today share your combination, and "1 in 1 million" means roughly 8,200 do. The "people on Earth" line does that conversion directly, multiplying 8.2 billion by your probability fraction.

Not entirely, and the calculator says so. Eye and hair colour share pigmentation genetics through the MC1R gene, so red hair and green eyes turn up together more often than plain probability predicts. Blood type is genuinely independent of both, since it's controlled by entirely separate genetic loci, and handedness has a genetic component that isn't strongly linked to pigmentation. The independence assumption works well enough for most combinations, a caveat appears on results that involve correlated traits, and the "1 in X" figure is best read as a close approximation.

AB negative, at approximately 0.5 to 1% of the global population (a little higher in some European populations), making it the rarest of the eight common ABO/Rh types. That scarcity matters in blood banking, where AB negative donors are actively recruited because their plasma suits any recipient and their red cells are needed for other AB negative patients. At the other end, O positive is the most common type worldwide at approximately 38% of people.

Because it needs two copies of a recessive variant. Red hair comes from variants in the MC1R gene that let pheomelanin (red and yellow pigment) dominate over eumelanin (brown and black), and since most MC1R variants are recessive both parents have to carry one for a child to be red-haired. Ireland and Scotland have the highest concentrations, with up to 13% of the population red-haired thanks to a founder effect in the Celtic population, while globally the figure is approximately 1 to 2%. The variants are ancient, and red hair was present in early European populations before dispersal diluted its frequency.

About 10.6% of people are left-handed on the best overall estimate in Papadatou-Pastou et al. 2020, a meta-analysis of 2.39 million participants, with a range of 9.3% to 18.1% depending on how strictly handedness is defined. The rough one-in-ten ratio holds across regions, though the figure shifts with the year of the study and with sex and ancestry. One caution applies to older adults. Schools in some countries forced left-handed children to switch during the 20th century, so self-reported left-handedness among older people is likely to run low.

Less precise than blood type, which comes from large clinical databases, because eye colour estimates rest on a mix of genetic population studies and surveys. The global green figure of 2% and the US/UK figure of 9% come from different sources using different methods, and genetic association studies tend to disagree with self-report surveys because people classify their own eyes inconsistently, with many "hazel" eyes carrying a lot of green. What you get here are the best estimates available, and true prevalence shifts a little with how the categories are drawn.

Not with the figures this calculator uses. The rarest combination it allows, heterochromia with red hair, AB negative blood, left-handedness and all three bonus traits, multiplies out at about 1 in 1.4 billion, and green eyes in place of heterochromia gives about 1 in 290 million. Several of those traits are correlated, so the real rarity is less extreme than even that arithmetic.