allele

An allele is one of the alternative spellings a gene can have. Imagine the same word printed in several editions of a book, mostly the same but differing by a letter here or there: each version is an allele, and which versions you carry shapes the corresponding trait.

Alleles sit at the same place, or locus, on a chromosome, but differ in their DNA sequence. Because chromosomes come in pairs in most cells, an individual carries two alleles of each gene, one inherited from each parent. They may be identical or different, and the combination determines how the gene behaves.

When the two alleles differ, their relationship matters. One allele may dominate, hiding the effect of the other, or the two may blend or both be expressed together. Across a whole population, many alleles of a single gene can exist, even though any one individual carries at most two of them.

It is worth remembering that no allele is inherently “good” or “bad” in isolation. An allele that causes disease when inherited from both parents may be harmless or even protective when inherited from one, as with the allele behind sickle-cell traits. Value depends on context and on which other alleles accompany it.