The British government has proposed rules for legalizing a fertility technique that uses material from three people. The practice would be restricted to a very few cases and only two of the people would be legally classed as parents.

The technique is called mitochondrial transfer and is designed to prevent the inheritance of some genetic disorders. Such disorders involve mitochondria (#9 above), a part of the outer layer of cells that powers cell growth.

The basis of the technique is to use traditional in vitro fertilisation to create two embryos: one with the mother’s egg and the father’s sperm, and the other with the father’s sperm and a donor’s egg. The donor embryo nucleus would then be replaced with the nucleus from the mother’s egg.

That creates an embryo with the parents’ combined genetic material and the donor’s mitochondria. Although that means the genetic material comes from all three people, hereditary characteristics would only come from the mother and father.

The UK’s Human Fertilization and Embryology Authority formally backed the technique from a medical perspective last year. The government has now finished developing proposals for the legal side of introducing the technique.

One key point in the proposals is that such treatment would be rare. Fertility clinics would need a fresh licence to carry it out, and a regulator would need to assess every proposed treatment case individually. It would only give the go-ahead if there was both a “significant” risk of a child getting a mitochondrial disease and if that disease would be “serious.” The BBC estimates as few as 10 cases a year would meet this threshold.

The rules also state that the donor would not be classed as having any family relationship to the child, and that the child would not have the legal right to information about the donor.

The proposed rules won’t take effect unless and until they pass a vote in Parliament, expected in early 2015. That’s likely to involve a heated debate about the ethical side of the technique.