" The darker side of this story is that parents in such (cot death) cases have often had their tragedy compounded by suspicion that they have killed their child. In one particularly sad case a few years ago an ‘expert’ witness informed a UK court that the chances of a couple’s first cot death through natural causes were 1 in 8,000, and, that, following the second cot death in the same family, the chances of that were 1 in 8,000 x 8,000 (i.e. 1 in 64 million). The mother went to prison because the ‘expert’ failed to understand that, if the first death, however unlikely, had a genetic cause, then the chances of the same defect affecting the next child were 1 in 4. For a recessive condition (see above), the assumption is that the two parents are carriers, each carrying one ‘bad’ copy and one ‘good’ copy of the gene. They are therefore unaffected themselves but both have a 50:50 chance of passing on the bad copy to their offspring. Hence a 25 per cent chance that the child will be affected and a 50 per cent chance that the child will be unaffected but also be a carrier like the parents. "
― , Enzymes: A Very Short Introduction