Which of the following is not a Mendelian Disorder?

The correct answer is Option (3) → Down's Syndrome

A Mendelian disorder comes from a mutation in a single gene; a chromosomal disorder comes from extra, missing or wrongly placed chromosomes.

Option (1) Haemophilia — A Mendelian disorder. Sex-linked recessive.

Option (2) Sickle-cell anaemia — A Mendelian disorder. Autosomal recessive.

Option (3) Down's Syndrome — Not a Mendelian disorder. It is a chromosomal disorder: an extra copy of chromosome 21.

Option (4) Phenylketonuria — A Mendelian disorder. Autosomal recessive.

Note: A similar question was also asked in CUET (UG) 2026 on 30-05-2026 (Slot 2) and on 14-05-2026 (Slot 1), and in CUET (UG) 2023 on 15-06-2023 (Slot 1).

NCERT Extract:
Mendelian Disorders – “Broadly, genetic disorders may be grouped into two categories – Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in the single gene. … Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc.”
Chromosomal Disorders – “The chromosomal disorders on the other hand are caused due to absence or excess or abnormal arrangement of one or more chromosomes. … For example, Down’s syndrome results in the gain of extra copy of chromosome 21. Similarly, Turner’s syndrome results due to loss of an X chromosome in human females.”