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.
Identify the X-chromosome linked disease from the given options.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Color blindness
A trait is X-linked when its gene lies on the X chromosome. A male has only one X chromosome, so a single faulty allele shows up in him. A female needs the faulty allele on both her X chromosomes. This is why such traits pass from a carrier mother to her sons.
(1) Thalassemia — Incorrect. Its genes lie on autosomes. It is an autosomal recessive disorder.
(2) Sickle-cell anaemia — Incorrect. Its gene is on an autosome, and the trait is autosomal recessive.
(3) Color blindness — Correct. It is a sex-linked recessive disorder. The mutated genes for red-green vision lie on the X chromosome, so it is far commoner in males.
(4) Phenylketonuria — Incorrect. It is an inborn error of metabolism that follows an autosomal recessive pattern.
Note: 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.
NCERT Extract:
Mendelian Disorders – “… Colour Blidness : It is a sex-linked recessive disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome.”