Trisomy for most autosomes is fatal, yet trisomy or even tetrasomy (four copies) of the X chromosome is not. What is the explanation for this difference?
A) The number of X chromosomes is always balanced by the number of Y chromosomes.
B) Only one copy of the X chromosome is active, regardless of the total number of X chromosomes.
C) The X chromosome does not carry any genes.
D) There is a mechanism to keep only two X chromosomes active, regardless of the total number.
Answer: B