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.
Haemophilia shows:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Sex linked recessive trait
The gene for haemophilia lies on the X chromosome. A heterozygous (carrier) female is not affected, because the normal gene masks the faulty one. She can pass the faulty gene to her sons, who have only one X chromosome. A female becomes haemophilic only if her mother is at least a carrier and her father is haemophilic.
(1) Sex linked recessive trait — Correct. It is carried on the X chromosome and it is hidden when a normal gene is present.
(2) Sex linked dominant trait — Incorrect. A dominant gene would show even in a carrier female. Carrier females of haemophilia are unaffected.
(3) Autosomal recessive trait — Incorrect. The gene is on the X chromosome and not on an autosome. Sickle-cell anaemia is the autosomal recessive example in the passage.
(4) Autosomal dominant trait — Incorrect. The gene is not on an autosome, and the trait is recessive.
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 3).
NCERT Extract:
Mendelian Disorders – …
