Three versions of one gene sort almost everyone into four blood types
Your blood type comes down to which versions of a single gene you inherited. Classical genetics recognises three, called IA, IB and i, which combine into six possible pairings and just four blood types. Those alternative versions of a stretch of DNA are called alleles, and they explain much of how traits pass down.
An allele is one variant of the DNA sequence found at a given spot, or locus, on a chromosome. Two alleles might differ by a single letter or by insertions and deletions thousands of base pairs long. Most variants change little, but some produce visible differences, from antibiotic resistance in bacteria to altered traits in plants and animals. The term shortens allelomorph, meaning other form, a coinage from the early 1900s credited to William Bateson and Edith Rebecca Saunders, two geneticists working in Britain.
Nearly all multicellular organisms are diploid at some stage, carrying two copies of each chromosome. Matching alleles make an organism homozygous at that locus; different ones make it heterozygous. When a heterozygote looks just like one of the homozygous forms, the allele responsible is called dominant and the hidden one recessive. The ABO blood gene has six common alleles in modern terms, and nearly every human's type reflects some mix of them.
Populations carry many alleles at each locus, and their frequencies predict genotype frequencies. In the simplest two-allele case under the Hardy–Weinberg principle, if one allele has frequency p and the other q, then p squared are homozygous for the first, q squared for the second, and 2pq are heterozygous. Some variants are null alleles, which do not work at all.
Dominance shapes inherited disease. Albinism, cystic fibrosis, phenylketonuria and Tay–Sachs disease appear only when someone inherits two recessive copies, whereas a single dominant allele is enough to cause Huntington's disease. Not all inheritance runs through sequence, either: in certain species chemical marks such as DNA methylation pass between generations, and these are called epialleles. Fungal geneticists adopted another word in 1990, idiomorph, for sequences sharing a locus across strains with no resemblance or shared ancestry.
Source: Allele
