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'Three-parent babies' explained

In November experts recommended a 'cautious adoption' of MRT. Credit: PA

Clinics can now apply to provide so-called "three-parent baby" treatment after the fertility regulator unanimously voted in favour of it.

The Human Fertilisation and Embryology Authority gave the green light to the pioneering treatment on Thursday, designed to combat inherited diseases.

Specialists in Newcastle will be ready to offer mitochondrial replacement therapy (MRT) to women who are in danger of passing on devastating and often fatal genetic disorders to their children from next spring.

Currently, such conditions affect about one in 10,000 births.

NHS England has also pledged £8 million over the next five years to support the treatment, where IVF children would receive a tiny amount of DNA from a third person other than their mother or father.

Creating a 'three-parent baby' involves removing the nucleus from one of the mother's eggs and inserting it into a donor egg that has had its own nucleus removed. Credit: PA

What is a "three-parent baby"?

The aim of creating "three-parent babies" is to replace abnormal genes in the energy-producing mitochondria in the body's cells.

Children inherit all their mitochondria from their mothers.

Mitochondria have their own DNA, called mitochondrial DNA (mtDNA), which is distinct from the genetic material within the cell nucleus which houses the vast majority of an individual's genes.

MtDNA only makes up around 0.1% of a persons DNA, but when it goes wrong it can prevent mitochondria working properly and the results can be catastrophic, leading to a wide range of potentially fatal conditions affecting vital organs, muscles, vision, growth and mental ability.

Mitochondrial Disease affects each person differently according to how many cells in the body are affected.

Some people can be carriers of faulty mtDNA but not experience ill-effects themselves.

MRT involves removing faulty mitochondria and substituting healthy versions from a donor and was made legal in Britain in 2015.

Babies born after MRT would effectively have three genetic parents. Their DNA would come from their mother and father, as well as a tiny proportion from an egg-donor.

MRT is carried out by transferring the genetic material that effectively encodes a baby's identity to the donor egg whose own nuclear DNA has been removed.

Two different techniques can be carried out for MRT, either before or after fertilisation.

The end result is the same - an embryo containing healthy mitochondria from the donor and nuclear DNA from the baby's mother and father.

What are the benefits of MRT?

As well as producing babies who have not inherited Mitochondrial Diseases, in theory mitochondrial replacement can also protect future generations.

Supporters back the procedure saying it allows women with a particular type of genetic disease to have children who are related to them.

Have "three-parent babies" ever been born?

Yes. Earlier this year the world's first "three-parent baby" was born to Jordanian parents, treated by a US-based team in Mexico.

Dr John Zang holds the world's first 'three-parent baby'. Credit: New Scientist

The baby's mother carries genes for Leigh syndrome, a disorder that affects the developing nervous system, and was responsible for the deaths of her first two children.

Are there any objections?

Animal and laboratory experiments suggest the procedure is safe, as do reviews carried out by the HFEA, however, critics argue that problems might only arise once the procedure is used to create human babies. For instance, they argue that replacing mtDNA might have more of an impact on personal traits than envisaged.

Critics also argue that environmental influences that alter the way genes work, may also have serious consequences for the health of babies.

Others are critical of the procedure on ethical grounds, saying the procedure is tantamount to genetic modification of humans, or even "playing God".