Single gene disorders with nonclassic inheritance

-  are rare & are briefly mentioned here.

-  can be classified into the following categories:

A. Diseases caused by mutations in mitochondrial genes.

    E.g. Leber hereditary optic neuropathy

B. Diseases associated with genomic imprinting

    E.g. Prader-Willi syndrome, Angelman syndrome

C. Diseases associated with gonadal mosaicism

Gonadal mosaicism can explain unusual pedigrees seen in some autosoma dominant disorders such as osteogenesis imperfecta in which phenotypically normal parents have more than one affected children. This cannot be explained by new mutations. Instead, it can be explained by gonadal mosaicism

D. Disorders caused by triplet repeat mutations

E.g. Fragile X syndrome 

-  is the second most frequent cause of hereditary mental retardation next to Down syndrome.

-   is clinically manifest in both males & females. In males, it is characterized by bilateral macro-orchidism (enlarged testes).

References

Bezabeh ,M. ; Tesfaye,A.; Ergicho, B.; Erke, M.; Mengistu, S. and Bedane,A.; Desta, A.(2004). General Pathology. Jimma University, Gondar University Haramaya University, Dedub University.