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.
The symptoms of short stature with small round head, furrowed tongue and partially open mouth in affected individual are found in:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Down’s syndrome
Down’s syndrome is caused by an extra copy of chromosome 21 (trisomy 21), so the person has 47 chromosomes. The affected person is short, with a small round head, a furrowed tongue and a partially open mouth. Physical, psychomotor and mental development is slow.
(1) Klinefelter’s Syndrome — Incorrect. The karyotype is 47, XXY. The person is tall, with some feminised features, and is sterile.
(2) Down’s syndrome — Correct. These are its typical features.
(3) Turner’s Syndrome — Incorrect. One X chromosome is missing (45, XO). The female is short, with rudimentary ovaries and poorly developed secondary sexual characters.
(4) Myotonic dystrophy — Incorrect. It is a single-gene disorder that is inherited as an autosomal dominant trait. It is not a chromosomal disorder with these features.
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:
Chromosomal Disorders –
Figure 4.16 A representative figure showing an individual inflicted with Down’s syndrome and the corresponding chromosomes of the individual