<prompt> Single gene disorder which does not follow Mendelian inheritance -Sickle cell anemiaDown syndromeFragile X-syndromeRetinoblastoma  <response>: 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. 