Allele
An allele is one of the alternative versions a gene can take at a given position on a chromosome. A person carrying two chromosomes of each pair therefore carries two alleles of each gene, which may be identical or different. An allele is called dominant when one copy is enough to show its effect and recessive when two copies are needed.
Why it matters
Almost every statement about genetic variation, in a population or in a person, is really a statement about which alleles are present and at what frequency.
Also written: alleles, allelic
Where it comes up
Read about Allele on Wikipedia
