An allele is one of two or more specific variants of a gene that exists at the same locus on a chromosome; a single allele has only one variant of a gene, while multiple alleles have more than two alleles for a gene, such in the case of blood type.

A specific variant of a gene is called an allele. A gene may have several alleles. A single allele means that there is only one variant of the gene. All alleles of the same gene exist at the same location, or locus, on a chromosome. Mendel states that a cell holds a gene containing two alleles, with one allele inherited from each parent. This means that only two alleles, one dominant and one recessive, could exist for a given gene.

Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may show at the population level such that many combinations of two alleles are observed. Multiple alleles exist when a gene has more than two alleles, such in the case of blood type. Blood type is governed by three alleles or genotypes (IA, IB, i) that dictate the phenotypes (A, AB, B, O). Since a cell can only hold two of these genes, different combinations that make up the blood types A, B, AB, and O are uniquely observed for each individual.

Multiple Alleles. Blood type is governed by three alleles or genotypes (IA, IB, i) that dictate the blood types or phenotypes (A, AB, B, O).

Practice Questions

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Key Points

• An allele is one of two or more specific variants of a gene that exists at the same locus on a chromosome; a cell holds a gene containing two alleles, with one allele inherited from each parent.
• A single allele means that there is only one variant of the gene; multiple alleles exist when a gene has more than two alleles, such in the case of blood type, where many combinations of two alleles are observed.

Key Terms

• allele: one of several alternative forms of the same gene occupying a given position on a chromosome
• single allele:  means that there is only one variant of the gene
• multiple alleles: where combinations of two alleles are observed in a genotype
• diploid: a nucleus containing two complete sets of chromosomes, one from each parent
• genotype: the characteristics coded for by alleles
• phenotype: relating to the observable characteristics of an individual resulting from the interaction of its genotype with the environment
• chromosome: a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
• locus: the position of a gene on a chromosome