Allele
An allele is one version of a DNA sequence at a particular genomic location. Alleles explain why individuals can carry different versions of the same gene or DNA segment, and they help connect inheritance, traits, genetic variation, and disease risk.
What an allele is
An allele is a version of a genetic sequence at a particular place in a genome. The location is often a gene, but the difference can be as small as one DNA base or as large as a longer segment. Alleles are one reason members of the same species can share the same genes but still differ in traits.
Alleles and genes
A gene is a functional stretch of genetic sequence. An allele is a version of that gene or sequence. For example, two people may both have the same gene at the same chromosome location, but their DNA letters at that location may differ slightly. Those alternative versions are alleles.
Inherited pairs
Humans and many other organisms are diploid, meaning most body cells carry paired chromosomes. For many autosomal locations, one allele comes from the egg and one from the sperm. If the two alleles are the same, the genotype is homozygous. If they differ, it is heterozygous.
Genotype and phenotype
A genotype is the allele combination an organism carries at one or more locations. A phenotype is an observable trait or measurable feature. Alleles can influence phenotypes by changing a protein, altering how much product a gene makes, affecting RNA, or changing gene regulation.
Dominant and recessive
In a simple dominant-recessive pattern, one allele can produce a visible effect even when paired with a different allele, while a recessive allele is usually seen only when both copies are recessive. These labels describe the relationship between genotype and phenotype; they do not mean one allele is stronger, better, or more common.
Beyond simple dominance
Many real traits do not fit a single dominant-versus-recessive pattern. Alleles can show incomplete dominance, codominance, variable penetrance, or effects that depend on other genes and the environment. The ABO blood group is a classic example in which A and B alleles are codominant.
Variants and mutation
Alleles arise when DNA sequence changes occur and are passed through populations. Some variants are inherited, some appear for the first time in a sperm, egg, or early embryo, and some occur only in body cells. Many variants have no obvious effect, while others influence traits, health, or adaptation.
Allele frequency
Population genetics often tracks allele frequency: how common a particular allele is in a population. Frequencies can change through mutation, natural selection, genetic drift, migration, and nonrandom mating. This makes alleles central to evolution as well as inheritance.
Why it matters
Alleles are the bridge between DNA differences and biological outcomes. They help explain family inheritance patterns, genetic testing results, blood types, inherited disorders, drug responses, ancestry studies, breeding, conservation genetics, and how populations change over time.