[1] Marked genetic heterogeneity is correlated to multiple levels of causation in many common human diseases including cystic fibrosis, Alzheimer's disease, autism spectrum disorders, inherited predisposition to breast cancer, and non-syndromic hearing loss.
[5] Alzheimer's disease is a complicated neurodegenerative disorder with multiple phenotypic subtypes, including clinical and preclinical, that result from different genetic origins.
[6] Current research on the amyloid cascade hypothesis has identified rare mutations in three genes that encode the amyloid precursor protein (APP), presenilin 1 (PS-1), and presenilin 2 (PS-2) that cause the autosomal dominant, early-onset form of familial Alzheimer's disease.
[7] Research has also discovered the association of a fourth allele, apolipoprotein E4 (ApoE4), in the development of late-onset and sporadic forms of the disease, although the pathology of its role is still largely unknown.
[15] 69 genes and 145 loci have been discovered to be involved in the genetic heterogeneity of non-syndromic hearing loss, and the phenotype of the disorder is largely associated with its pattern of inheritance.