Alternative forms of a gene at the same locus (position on a chromosome)
For instance, at the ABO gene locus on chromosome 9, there are three main alleles in the population - A, B and O. An individual inherits two alleles at the ABO blood group locus - one from each parent. The two alleles together are responsible for a person's ABO blood group type, which is very important to know for blood transfusions. The different possible combinations of alleles give rise to the A, B, AB and O blood groups.
It is not usual to talk about "alleles" when discussing genetic conditions with patients. Health professionals usually refer to the "usual form of the gene" and the "gene containing a change which has caused the condition." Patients do not usually like the use of the term "mutation" when talking about a gene which has been changed from the usual form.
The term "allele" may well be seen on molecular genetics reports which describe, in technical language, a mutation which may have been found.
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