When I watched Gattaca, I assumed that the 6-fingered piano artist wasn't genetically bread, but that he was conceived through sex. Apparently the gene coding for six fingers is dominant. If he was an "invalid" or "love-child", it's quite possible that he would have been born with six fingers. I thought it was a nice touch by the director, showing how an unchosen mutation in a naturally born child resulted in a genetic advantage over genetically designed children.