They purchased the sperm from a sperm bank,. All reputable banks screen for safety - it was part of the trade-off in choosing a sperm bank over a friend and a turkey baster.
My daughter's biological father donated to a local sperm bank. Through genetic testing, she has discovered her bio-dad and a half dozen half-siblings locally (as well as the three in the donor's family). She just went to the baby shower for the oldest of the half-siblings in the traditional family, and is in fairly regular contact with the donor's wife, who has the same inclination toward alternative medicine as my medically intensive daughter.
The sperm is donated, frozen for a period of time, at the end of which the father is tested for things like AIDS (which might be present, but not yet positive at the time of the donation). It is not available for use until after the secondary testing of the donor. There is an extensive questionnaire health examination and questionnaire. Part of it I knew at the time - part of it I learned at birth when they wanted to screen her for a number of conditions because the father was listed as "unknown."
Once they knew she was conceived by donor insemination, they skipped that screening because the donor insemination screening was more extensive than what they would obtain from a known father.
They don't test for rare genetic mutations not present in the donor or his then-existing family. I haven't read the article yet, but I would guess that this mutation was discovered when either the donor's traditionally conceived child was diagnosed with the syndrome, or one of his donor children was and they tested any remaining donations. Because he was a carrier who did not have the condition, there would be no reason to test for it.
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