Complex sentence: Since women have two X chromosomes, if one X chromosome has the non-working gene, the second X chromosome will have a working copy of the gene to compensate, because of this ability to compensate, women rarely develop symptoms.
Split into simpler sentences: Since women have two X chromosomes, if one X chromosome has the non-working gene, the second X chromosome will have a working copy of the gene to compensate. Because of this ability to compensate, women rarely develop symptoms.