Suppose you have a woman who knows that she comes from a family with a history of
Duchenne muscular dystrophy, a debilitating disease which kills fairly young (typically earlier than age 30) and leaves the sufferers crippled for most of their lives. She knows this because her full-blooded brother has the condition. However, she doesn't know if she's a carrier herself or not. There is a fifty percent chance that she inherited the carrying X chromosome from her mother. The gene for Duchenne's can't do her any harm, so perhaps she isn't interested in learning if she carries it or not. That's fine. But what if she wants to have children?
There is a 12.5% chance that any given child she has will have Duchenne's: 50% that she's a carrier in the first place, 50% that the child in question is a boy, and 50% that the boy gets the wrong X chromosome. (She also has a 12.5% chance of having a girl who carries the disease herself and will have to deal with this same problem if she wants children when she grows up.)
Is it ethical for this woman to have children without getting a genetic test, or employing preimplantation genetic diagnosis (fertilizing several eggs and keeping ones that are genetically healthy)? If she gets tested, and finds that she does not carry the disorder, then clearly there's no dilemma here; but what if she tests positive for the gene?
For comparison, let's say the woman's neighbor is secretly a witch and she wants a magic baby. She has the same aversions to getting a baby via nonmagical means that the potential carrier of Duchenne's has to getting a baby through adoption. Anyway, the witch has a magic wand which can generate magic babies. The wand has a second feature, which is to give a nonmagical human infant muscular dystrophy every time it generates a baby. (The wand has a safety feature which prevents the condition from being bestowed upon any children who have parents less accommodating and supportive than the carrier next door would be.) There is a fifty percent chance that this feature of the wand is not working at all; a fifty percent chance that if it is working, it will malfunction at the last moment and spare the child it targets; and a fifty percent chance that if it's working and stays functional throughout the waving of the wand, it will strike a female infant and make her a carrier instead of striking a male and making him a sufferer. (Please ignore for the purposes of this analogy that a magic-wand-induced genetic abnormality will not be anticipated because it will lack a family history.)
Would you say she was within her rights to wave that wand? Without checking to see if the second feature worked with her handy dandy wand-checking spell? Fill in the rest of the analogy as necessary to make it correlate. For instance, if taking away somebody's magic wand sounds like a lesser intrusion than sterilization, just stipulate that witches have as close a tie to their wands and as difficult a separation process therefrom as non-witches and their relevant body parts. If you want to involve paternity, the witch needs a warlock assistant to wave the wand. Whatever.
If you would not like to let the witch wave her wand, but you would like to say that the carrier of Duchenne's can freely breed without taking a genetic test, then you have very worrisome intuitions about people's prerogatives over their own children or you have weird opinions about genetics. (If you'd let them both go about baby-getting by whatever method they chose there is something seriously wrong with you.) In general, there are some things it is acceptable to inflict on one's own children that one would not perpetrate on another's, but these things are along the lines of dictating bedtimes.
A parent who physically abused their already-born offspring to the point of causing the same debilitation and eventual death that Duchenne's causes would be locked up. Even if they did it only to their sons and only if that unlucky son flipped a coin heads up twice in a row.
Why are parents with known risks of such horrifying diseases permitted to inflict that risk on their children?