Allelic heterogeneity means that different mutations within a single gene locus (forming multiple alleles of that gene) cause the same phenotypic expression. For example, there are over 1000 known mutant alleles of the CFTR gene that cause cystic fibrosis.> Locus heterogeneity is the phenomenon in which different mutations at the same locus causes a similar phenotype. For example, b-thalassemia may be caused by several different mutations in the b-globin gene.> Allelic heterogeneity should not be confused with locus heterogeneity in which a mutation at a different gene causes a similar phenotype.> Nor should it be confused with phenotypic heterogeneity in which a mutation within the same gene causes a different phenotype.> Allelic Loss: The loss of one allele at a specific locus, caused by a deletion mutation; or loss of a chromosome from a chromosome pair. It is detected when heterozygous markers for a locus appear monomorphic because one of the alleles was deleted. When this occurs at a tumor suppressor gene locus where one of the alleles is already abnormal, it can result in neoplastic transformation.">