Occasionally, I come across the term “designer baby,” and I am reminded that some parents now have the option to screen or modify the genes of their unborn children to ensure or avoid certain traits. It always gives me a feeling of unease. Obviously, I did not take this route — partly because, at least with my son, I never had to actually make the decision. My third child, Eliza, was a late midlife accident. I chose to have her despite the possibility she would have XLH, but would I have made the same decision in a planned pregnancy or if given a choice much earlier in the process? I can’t help suspecting that because of advances in genetic mapping, genetic testing, the sheer range of prenatal choices, chances are that in a generation or two, there will be no one in the world who has XLH, no one who looks like me or my children — at least not in the so-called developed world — and I don’t know how to feel about that.