But some disorders pose greater challenges. Down’s syndrome, for example, happens when people are born with three copies of the 21stchromosome, rather than the usual two. This condition, called trisomy, leads to hundreds of abnormally active genes rather than just one. You cannot address it by correcting a single gene. You’d need a way of shutting down an entire chromosome.
But half of us do that already. Women are masters of chromosomal silencing.
Women are born with two copies of the X chromosome, while men have just one. This double dose of X-linked genes might cause problems, so women inactivate one copy of X in each cell.