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 X chromosome does not carry any genes. b) Only one copy of the X chromosome is functional, regardless of the total number of X chromosomes. c) There is a mechanism to keep only two X chromosomes functional, regardless of the total number. d) The number of X chromosomes is always balanced by the number of Y chromosomes.

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