A genetic term that means “an alternate form of a gene.” Alleles account for genetic variation, and alleles of a gene are present at a defined location (“locus”) on a particular chromosome. For example, the ABO gene locus has three main alleles that can be inherited on chromosome 9: A, B, and O. The combination of the allele inherited from one parent with that inherited from the other determines the person’s final ABO type. For example, if a person inherits an A allele from his dad and a B allele from his mom, his ABO type will be “AB.” This can get very convoluted with multiple possible alleles, as is seen in the Rh blood group system.

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