Inheritance and disorders
Read the passage carefully and answer the given questions.
Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. In haemophilia, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this, in an affected individual a simple cut will result in non-stop bleeding. The heterozygous female for haemophilia may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least carrier and the father should be haemophilic. Sickle-cell anaemia is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene. The disease is controlled by a single pair of allele, HbA and HbS. 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 cycle results in the gain or loss of a chromosome, called aneuploidy.
Which one of the following disorders is an example of point mutation?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Sickle cell anemia
A point mutation is a change of a single base pair in the DNA. In sickle-cell anaemia one base changes in the gene for the beta chain of haemoglobin. This is the textbook example of a point mutation.
(1) Sickle cell anemia — Correct. The change from GAG to GUG at the sixth codon of the beta globin gene is a single base substitution.
(2) Haemophilia — Incorrect. Haemophilia is the sex linked recessive disease in which a protein of the blood clotting cascade is affected. It is not given as the example of a point mutation.
(3) Turner’s syndrome — Incorrect. This is a chromosomal disorder. One X chromosome is lost, so the cells have 45 chromosomes.
(4) Down’s syndrome — Incorrect. This is a chromosomal disorder. There is an extra copy of chromosome 21.
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) 2023 on 05-06-2023 (Slot 1).
NCERT Extract:
Mutation – “… Chromosomal aberrations are commonly observed in cancer cells. In addition to the above, mutation also arise due to change in a single base pair of DNA. This is known as point mutation. A classical example of such a mutation is sickle cell anemia.”