Allele - A Genetics Definition

An allele is one of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. Darryl Leja / NHGRI

An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring through sexual reproduction. The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.

Examples of Dominant and Recessive Alleles

Diploid organisms typically have two alleles for a trait.

When allele pairs are the same, they are homozygous. When the alleles of a pair are heterozygous, the phenotype of one trait may be dominant and the other recessive. The dominant allele is expressed and the recessive allele is masked. This is known as complete dominance. In heterozygous relationships where neither allele is dominant but both are completely expressed, the alleles are considered to be co-dominant. Co-dominance is exemplified in AB blood type inheritance. When one allele in not completely dominant over the other, the alleles are said to express incomplete dominance. Incomplete dominance is exhibited in pink flower color inheritance in tulips.

Multiple Alleles

While most genes exist in two allele forms, some have multiple alleles for a trait. A common example of this in humans is ABO blood type. Human blood type is determined by the presence or absence of certain identifiers, called antigens, on the surface of red blood cells.

Individuals with blood type A have A antigens on blood cell surfaces, those with type B have B antigens, and those with type O have no antigens. ABO blood types exist as three alleles, which are represented as (IA, IB, IO). These multiple alleles are passed from parent to offspring such that one allele is inherited from each parent.

There are four phenotypes (A, B, AB, or O) and six possible genotypes for human ABO blood groups.

  Blood Groups    Genotype
           A  (IA,IA) or (IA,IO)
           B  (IB,IB) or (IB,IO)
          AB  (IA,IB)
           O  (IO,IO)

The alleles IA and IB are dominant to the recessive IO allele. In blood type AB, the IA and IB alleles are co-dominant as both phenotypes are expressed. The O blood type is homozygous recessive containing two IO alleles.

Polygenic Traits

Polygenic traits are traits that are determined by more than one gene. This type of inheritance pattern involves many possible phenotypes that are determined by interactions among several alleles. Hair color, skin color, eye color, height, and weight are all examples of polygenic traits.The genes contributing to these types of traits have equal influence and the alleles for these genes are found on different chromosomes. A number of different genotypes arise from polygenic traits consisting of various combinations of dominant and recessive alleles. Individuals inheriting only dominant alleles will have an extreme expression of the dominant phenotype. Individuals inheriting no dominant alleles will have an extreme expression of the recessive phenotype.

Individuals inheriting different combinations of dominant and recessive alleles will exhibit varying degrees of the intermediate phenotype.