Cells from the baby's bone marrow or umbilical cord are used to treat the older child.

BBC: Embryology Bill: The key points

Late last year, Jack and Lisa Nash, a Colorado couple, opted for the high-tech approach to avoid their new baby suffering from Fanconi anaemia, a rare inherited bone-marrow disease, which plagues their daughter.

ECONOMIST: Gene genies

It is a particular gamble if the child is conceived partly to provide a bone marrow graft for a brother or sister with a nasty genetic disease: not only might the new baby turn out be a poor match for transplantation, but might also have inherited the same disorder as his or her sibling.

ECONOMIST: Gene genies