Using the sperm and egg taken from a couple, scientists create embryos by in vitro fertilisation (IVF). Scientists examine these two or three-day old embryos to find out which genes for disease or other features are present in each embryo.
The couple can then choose which embryo, usually the healthiest, to put into the mother to grow into a baby.
Scientists are able to select embryos that are free from life-threatening or mild genetic diseases. They can also select male or female embryos. But what characteristics should people be allowed to choose for their children?