Genetic disorders
Read the following passage carefully and answer the given questions.
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. It is important to mention here that such Mendelian disorders may be dominant or recessive. By pedigree analysis one can easily understand whether the trait in question is dominant or recessive. The chromosomal disorders on the other hand are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Down’s syndrome, Turner’s syndrome, Klinefelter’s syndrome are common examples of chromosomal disorders. Mutation is defined as change in the genetic material. A point mutation is a change of a single base pair in DNA. Sickle-cell anaemia is caused due to change of one base in the gene coding for beta-chain of hemoglobin. The total number of chromosomes in a normal human cell is 46 (23 pairs). Out of these 22 pairs are autosomes and one pair of chromosomes are sex chromosome. Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s) called aneuploidy. Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and this phenomenon is known as polyploidy.
Which one of the following is not a Mendelian disorder?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Down’s syndrome
Genetic disorders are of two kinds. A Mendelian disorder is caused by a change (mutation) in a single gene and is passed on as Mendel’s laws describe.
(1) Thalassemia — A Mendelian disorder. A mutation in the gene for a globin chain of haemoglobin causes it.
(2) Sickle-cell anaemia — A Mendelian disorder. A change of one base in the gene for the beta-chain of haemoglobin causes it.
(3) Color blindness — A Mendelian disorder. A mutation in a single gene on the X chromosome causes it.
(4) Down’s syndrome — Not a Mendelian disorder. It is a chromosomal disorder. The cause is an extra copy of chromosome 21, not a fault in one gene.
Note 1: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
Note 2: A similar question was also asked in CUET (UG) 2026 on 14-05-2026 (Slot 1), and in CUET (UG) 2023 on 15-06-2023 (Slot 1), on 13-06-2023 (Slot 3) and on 26-05-2023 (Slot 3).
NCERT Extract:
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. Similarly, Turner’s syndrome results due to loss of an X chromosome in human females.”
Mendelian 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.”