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Question 582
Genetics → General
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Question
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Which interpretation most accurately distinguishes the smear finding from the underlying state of the X chromosomes?
Question Stem
A buccal smear from a normal woman contains 100 assessable somatic-cell nuclei. A Barr body is visible in 24 nuclei. A student concludes that X inactivation therefore occurs in only 24% of her buccal cells and that the remaining cells retain two active X chromosomes.
Options
A
The student's interpretation is correct because every inactive X chromosome must be visible in this smear
B
The 24 visible bodies identify cells in which both X chromosomes have been inactivated
C
Barr-body visibility underestimates the number of cells with an inactive X; one X is inactive in each normal female somatic cell
D
The visible Barr bodies represent active X chromosomes, so the count measures X activation rather than inactivation
E
Buccal cells follow the germ-cell exception and retain two active X chromosomes despite their visible Barr bodies
Explanation
One X chromosome is inactive in each normal female somatic cell, but the condensed Barr body is not identified in every nucleus on a buccal smear. It may be seen in up to 30% of cells, so visibility in 24 of 100 cells is compatible with somatic X inactivation throughout the sample. Failure to see a Barr body in the remaining nuclei does not establish two active X chromosomes. Buccal cells are somatic cells and do not fall within the germ-cell exception.
Option Validity
A) The observed proportion of visible Barr bodies is not the proportion of somatic cells that have undergone X inactivation.
B) A Barr body represents an inactive X chromosome; the normal female somatic pattern is one inactive X with the other remaining active.
D) The Barr body is condensed inactive X chromatin, not a marker of the active X.
E) The exception concerns germ cells, which require both X chromosomes; it does not apply to the buccal somatic cells being examined.
Further Reading
Saxena R. Textbook for MRCOG-1: Basic Sciences in Obstetrics and Gynaecology. Jaypee Brothers Medical Publishers; 2016. Chapter 8, Genetics: X Chromosome and The Lyon Hypothesis. Printed pages 257–258.