Britain could be the first country to legalise the controversial mitochondrial transfer technique. What does the procedure entail, and what will it mean for parental rights?

Britain will be on the path to becoming the first country in the world to permit the creation of “three-parent” babies if MPs vote in favour of changing the law on Tuesday.

The procedure replaces a small amount of faulty DNA in a mother’s egg with healthy DNA from a second woman, so that the baby would inherit genes from two mothers and one father. The idea is to prevent certain genetic diseases being passed on to children. Most experts are in favour but a handful have raised concerns, as has the Church of England.

British MPs have been given the right to vote with their consciences. Under current UK law, the procedure is banned because genetically altered embryos cannot be implanted into a woman. If MPs in the House of Commons approve the change in law, the decision will pass to the House of Lords for a vote at the end of February - and if the Lords agree the Human Fertilisation and Embryology Authority could license clinics to perform the procedure as soon as this autumn – and the first babies could be born in 2016.

What is the problem the procedure solves?

The procedure replaces a small amount of faulty DNA in a mother’s egg with healthy DNA from a second woman. Photograph: AFP/Getty Images

A small number of children each year are born with faults in their mitochondrial DNA which can cause diseases. Mitochondria are small structures that sit inside our cells and provide them with energy. They have their own set of 37 genes which are separate from the 20,000 or so genes that shape who we are.

How do the diseases affect people?

Mitochondrial diseases tend to strike in childhood and get steadily worse. They often prove fatal before adulthood. The parts of the body that need most energy are worst affected: the brain, muscles, heart and liver. Conditions include Leigh’s disease, progressive infantile poliodystrophy and Barth syndrome. Faulty mitochondria have also been linked to more common medical problems, including Parkinson’s, deafness, failing eyesight, epilepsy and diabetes.

What objections do people have to the procedure?

Mitochondrial transfer passes on genetic changes from one generation to another. That raises ethical concerns because any unexpected problems caused by the procedure could affect people who are not yet born, and so cannot give their consent to have the treatment. Mitochondria are not completely understood, and the DNA they hold might affect people’s traits in unknown ways. For that reason, some scientists believe mitochondria should be better understood before the procedures are legalised. The Church of England says it is not opposed in principle, but wants to see more scientific research and debate on the ethics, safety and efficacy before the law is changed.

Are there other religious objections?

The Catholic church opposes one form of mitochondrial transfer, called pronuclear transfer, because a fertilised egg from the mother is destroyed in the process. Catholic ethicists have also complained that mitochondrial transfer introduces a “rupture” between mother and father and “dilutes parenthood”.

Is ‘three-parent’ babies a good description of children born to the procedure?

No. Women who donate their mitochondria would remain anonymous and have no rights over the child. They are not involved in the child’s upbringing. On a genetic level, all of the 20,000 genes on the child’s 23 pairs of chromosomes come from the child’s mother and father. The donor only contributes DNA that sit in the mitochondria, less than 0.2% of the total.

Will a change in law allow ‘designer’ babies?

No. Human characteristics, such as eye and hair colour and other defining traits, are controlled by DNA in the cell nucleus. The procedure does not change this “nuclear” DNA. The ban on altering nuclear DNA remains in place.

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How common are mitochondrial disorders?

About one in 200 children are affected by mitochondrial diseases. Many of these will have only mild symptoms, or problems that emerge later in life. But about one in 6,500 have serious illnesses that can cut their lives short. There are no cures for mitochondrial disorders.

How are mitochondrial disorders passed on?

Even though everyone has mitochondria in their cells, only mothers pass them on to their children. Some women may carry faulty mitochondria without knowing. A woman might have so few faulty mitochondria that she herself has few or no symptoms. But her eggs will carry various amounts of faulty mitochondria, so she can still pass on a mitochondrial disease. For affected women, whether or not a child develops a disease is down to the biological lottery of which egg is fertilised.

Women who donate their mitochondria would remain anonymous and have no rights over the child. Photograph: Alamy

How can mitochondrial diseases be prevented?

There are several options for affected women who want to have children. They can adopt, or have IVF with eggs donated from a healthy woman. There is no risk in these cases, because the children have none of the mother’s DNA. For women who want genetically related children there are other options. They can have IVF with pre-implantation genetic diagnosis, which can pick up mitochondrial mutations. If the parents are willing to abort an affected foetus, they can have chorionic villlus sampling at 10-12 weeks, or amniocentesis at 14-20 weeks. Both can pick up mitochondrial diseases, but they carry a small risk of miscarriage. For some women, these tests are no use because all of their eggs carry substantial mitochondrial mutations.

What are the new procedures that could prevent the diseases?

Scientists have developed two techniques to stop mitochondrial diseases being passed from mother to child. The first is called mitochondrial spindle transfer (MST). In this, doctors use standard IVF procedures to collect eggs from the mother. They take the nucleus from one of the eggs and drop it into a healthy donor egg that has had its own nucleus removed. The reconstituted egg contains all the normal genes from the mother, but her faulty mitochondria are replaced by those from the healthy donor. The egg is then fertilised with the father’s sperm. The resulting embryo has the usual 23 pairs of chromosomes that hold the mother and father’s DNA, but the 37 mitochondrial genes, about 0.2% of the total, come from a third person, the donor. The second procedure is called pronuclear transfer. It is similar to MST, but both the mother’s and donor’s eggs are fertilised first with the father’s sperm. Before the eggs divide into early stage embryos, the parents’ chromosomes are removed from the mother’s fertilised egg and placed into the donor egg, which has had its own chromosomes removed.

Is mitochondrial transfer safe and effective?

Both procedures have been tested in animals and resulted in healthy offspring. Good results have also been seen in human cells, but treated embryos have not been implanted into a woman to achieve a pregnancy. A review of work on mitochondrial transfer by the HFEA’s independent scientific panel concluded there was no evidence the procedures were unsafe.