This is probably Robin Cook's Mutation.
Victor Frank, and his wife Marsha, are unable to have a second child due to Marsha's infertility. They turn to surrogacy as an alternate method of conception. Victor, an obstetrician-gynaecologist and owner of the biochemical company Chimera Inc., injects the egg implanted in his wife with an agent called Nerve Growth Factor (NGF) into chromosome six, which causes the baby to grow more neurons than usual, as a result making them super intelligent. Their son, VJ, is born a genius. He is able to talk in six months and read in thirteen.
Several years later, VJ's brother, David, and nanny, Janice, both die of an unexplainable rare form of liver cancer. At about three, VJ experiences a drop in intelligence, leading Victor to think his experiment is a failure...
....
One year later, a mother brings in her teenage daughter and her daughter's child to her office. Marsha surmises from the fact that the 18-month child is reading a medical journal, something VJ did, and his ice blue eyes, a trait VJ had, that this child is the "failed" zygote. She decides she will have to go through another VJ-like experience, "with Joe's help and end forever the nightmare her husband had begun".
