Perfect People — Peter James

In Peter James' "Perfect People," when a young couple learn they are both carriers of a gene likely to give their children a rare genetic disease, they visit a secret clinic for a 'designer baby.' But it does not all work out as planned...

After the death of their four-year-old son from a rare genetic disorder, John and Naomi Klaesson learn that if they have another baby there's a high risk the same thing will happen. Desperate to avoid going through the pain of losing another child, the couple visit a clinic located on a converted cruise ship in international waters, free from any legal restraints, run by a maverick geneticist, Dr. Leo Dettore. For $400,000, they can choose all the genes of their child--literally have a designer baby. However, they don't want half the choices on offer, they just want a healthy child. Dettore tells them that if they don't do all they can, they will risk their child being born in a genetic "underclass" because so many parents will be enhancing theirs...

On returning home to LA, their first shock is that instead of being pregnant with the son they wanted, Naomi finds she is having twins. When John, drunk, admits to a journalist they are having a designer baby, it gets into the press and they start to be hunted by a group of religious fanatics who are violently against tampering with nature or God's will.... They flee to the UK to get away, and John takes up a research post in Sussex. The kids are born a boy and girl, and very soon he and Naomi realise they are not just bright, they are unnervingly intelligent--even at a few years old already smarter than their parents in many ways. The teacher asks them to take them out of school as they frighten other children. One morning, John finds they have killed and postmortemed their pet guinea pigs, and they don't understand why he is angry at them. So far as they were concerned, they were doing research.