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Question 573
Genetics → General
Classification
Question
SSBAOptions
Explanation
The cells can retain the same two X chromosomes yet differ in which one remains active. Random inactivation may affect either the maternal or the paternal X. In cells with the normal paternal X inactive, the abnormal maternal X remains active and can account for manifestations in a carrier. Cells with the abnormal maternal X inactive express the normal copy. Lyon inactivation applies to somatic cells; germ cells are an exception.
Option Validity
A) The stated distribution reverses the distinction: Lyon inactivation applies to somatic cells, whereas germ cells require both X chromosomes.
B) Universal inactivation of the abnormal X would not explain the minority of cells expressing its gene.
D) Either the maternal or paternal X may be inactivated; a paternal-only rule does not explain the observed mixture of active copies.
E) Expression from the abnormal X requires it to remain active. Inactivation of both X chromosomes is not the mechanism described by the Lyon hypothesis.
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.