Question

Down’s syndrome is caused by the presence of an extra chromosome number.
(a) X
(b) Y
(c) 21
(d) 22

Answer 1

This chromosome is the smallest human chromosome spanning about 48 million base pairs and having 1.5 to 2 percent of the total DNA cells. This is the second human chromosome to be fully sequenced containing 200 to 300 genes that provide instruction for protein production.

Complete step by step answer:
Down syndrome is one of the most common chromosome abnormalities named after John Langdon Down, the British doctor who fully described the syndrome. Down syndrome also known as trisomy 21 or Mongolism which is a genetic disorder caused by the presence of one extra copy of chromosome 21 rather than two copies each one from both the parents.
- The down syndrome is usually related to physical growth delays, characteristic facial features and mild to moderate intellectual disability indicates trisomy of 21.
- Trisomy 21 is caused by a meiotic nondisjunction event where a gamete is produced with an extra copy of the chromosome of 21, the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo now has 47 copies with three copies of chromosome 21.
- Having extra copies of genes on chromosome 21 disrupts the course of normal development, causing the characteristics of Down syndrome.
- People suffering from Down’s syndrome experience a gradual decline in thinking ability as they age which can also lead to an association with increased risk of developing Alzheimer disease soon.
So, the correct answer is, ‘(c) 21’.

Note:
- This trisomy occurs in egg cells but occasionally occurs in sperm cells.
- Humans who inherit an unbalanced translocation involving chromosome 21 may have extra genetic material from chromosome 21 which causes mongolism.
- Mosaic Down syndrome is not inherited as it occurs at random events during cell division early in fetal development.