Single gene disorder which does not follow Mendelian inheritance -
Transmission of certain single-gene disorders does not follow classical Mendelian principles.
This group of the disorder can be classified into the following categories -


Diseases caused by triplet - repeat mutation, e.g. fragile - X syndrome, Huntington disease, & others.
Disorders caused by a mutation in mitochondrial genes, e.g. labour hereditary optic neuropathy.
A disorder associated with genomic imprinting.
Disorders associated with gonadal mosaicism.
Sickle cell anemia
Down syndrome
Fragile X-syndrome
Retinoblastoma