RIYADH, Saudi Arabia, April 24— When she was 17, marrying age for a Saudi girl, Salha al-Hefthi was presented with a husband.
She was lucky, her parents told her when they planned the wedding, that she was to marry such a good man, a man from her own tribe, a man who would care for their children and make a good living. He was the son of her father's brother -- her first cousin -- and everyone, including the bride, agreed that ''a first cousin was a first choice,'' she said.
The couple had two healthy boys, now 22 and 20, but their third child, a girl, was born with spinal muscular atrophy, a crippling and usually fatal disease that was carried in the genes of both parents. Their fourth, sixth and seventh children were also born with the disorder.