This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Single gene disorder which does not follow Mendelian inheritance - 
Select and explain. 
A. Sickle cell anemia, B. Down syndrome, C. Fragile X-syndrome, D. Retinoblastoma
C: Fragile X-syndrome
Answer: C: Fragile X-syndrome 
Explanation: 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.