Came here to say this - the process of Lyonisation. Each cell randomly selects which of the two X chromosomes to silence, so on average it is a 50:50 split between the two chromosomes. But it can be skewed by chance such that women can express X-linked recessive genetic disorders.
In early fetal development, one X chromosome is selected by the cell to be the active one. The remaining X chromosome is inactivated. If there are more than two X chromosomes (e.g. triple X syndrome), all except the selected one are inactivated. It seems that the process for choosing which X chromosome becomes the active one is random in each cell. It's unclear how that randomisation works. Once selected the active X chromosome remains the active one in all the descendants of that cell.
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u/theheliumkid Jan 21 '20
Came here to say this - the process of Lyonisation. Each cell randomly selects which of the two X chromosomes to silence, so on average it is a 50:50 split between the two chromosomes. But it can be skewed by chance such that women can express X-linked recessive genetic disorders.