Which one of the following traits is not a Mendelian disorder?

The correct answer is Option (2) → Down’s syndrome

Genetic disorders are of two kinds. Mendelian disorders are caused by a change (mutation) in a single gene, and they follow Mendel’s pattern of inheritance. Chromosomal disorders are caused by the absence, excess or wrong arrangement of one or more chromosomes.

(1) Haemophilia — A Mendelian disorder. It is a sex-linked recessive disorder in which blood does not clot properly.

(2) Down’s syndrome — Not a Mendelian disorder. It is a chromosomal disorder, caused by an extra copy of chromosome 21 (trisomy 21).

(3) Cystic fibrosis — A Mendelian disorder. It is caused by a mutation in a single gene.

(4) Phenylketonuria — A Mendelian disorder. It is an autosomal recessive disorder. The person lacks the enzyme that changes phenylalanine into tyrosine.

Note: A similar question was also asked in CUET (UG) 2026 on 30-05-2026 (Slot 2), and in CUET (UG) 2023 on 15-06-2023 (Slot 1) and on 26-05-2023 (Slot 3).

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. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. 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. Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. For example, Down’s syndrome results in the gain of extra copy of chromosome 21.”