Question

Number of Barr bodies in human female suffering from Down's Syndrome would be

• A. 3
• B. 1
• C. 2
• D. Zero

Answer 1

Answer: (B)

1

The presence of full or partial extra three copies of chromosome 21 causes Down syndrome. It is called as trisomy 21 and is represented as 45+XY or 45+XX. As we know that human females have two copies of X chromosome while the male has only single copy. To compensate the dosage of X-linked genes in human male and female, one X-chromosome in females is deactivated and is present as darkly stained body near nuclear envelope. The number of Barr bodies is always one less than the total number of X chromosomes. Since, Down syndrome does not affect the number of X-chromosome; a female with this syndrome will have 1 barr body. This is because the human female has two X-chromosome. To have 3, 2 and 0 Barr body, a female should have 4, 3 and 1 X chromosome respectively; this makes option A, C and D wrong. The correct answer is A.