The UK has legalized a new three-parent in vitro fertilization technique that uses DNA from two mothers and a father to make one baby. Newcastle University is the seat of the action, with UK regulatory bodies avowing that this treatment will only be available on a case-by-case basis.

While the first so-called three-parent baby was born last year, the development in the UK is noteworthy for being the first official sanctioning of the IVF procedure, offering a blueprint for the technique’s regulation as well as paving the way for its increased availability; the procedure that led to the birth of the baby boy in 2016 was performed in Mexico, which has no regulations in place.

This three-parent IVF treatment is actually called mitochondrial replacement therapy – it is an in vitro fertilization technique that involves replacing risky mitochondrial DNA in the mother’s genes with healthy mitochondria from a donor, allowing the child to avoid inheriting certain diseases that affect roughly 1 in 6,500 children.

Mitochondria are small cellular structures that generate energy to power the body. They have their own DNA, which dictates their function and energy production. Mitochondrial DNA is inherited only from mothers’ eggs and it does not determine our personality or appearance – that is determined by DNA present in the cell’s nucleus, or nuclear DNA. Less than 1% (or 37 of 20,000) of our DNA is mitochondrial.

There is currently a ban on tampering with nuclear DNA.

Mitochondrial diseases tend to manifest in early childhood and only get worse. Some children do not survive infancy due to them. Conditions like progressive infantile poliodystrophy, Barth syndrome and Leigh’s diseases, as well as diseases affecting the brain, muscles, heart and liver are due to mitochondrial DNA. Diabetes, heart issues, epilepsy, Parkinson’s, failing eyesight, muscle weakness are also passed on by mitochondrial genes. Most women would need to undergo genetic testing to confirm whether their mitochondrial DNA carries markers for these diseases, before considering mitochondrial replacement therapy.

There are currently two experimental mitochondrial DNA replacement techniques that have been consistently successful. One is called maternal spindle transfer (MST). In this, doctors use IVF to collect eggs from the mother, remove the nucleus from an egg and transfer it into a healthy donor egg sans nucleus. Now, the donor egg with the mother’s nucleus is fertilised with sperm to make an embryo.

The second procedure, pronuclear transfer (PNT), involves fertilising both mother’s and donor’s eggs with the father’s sperm. Before the eggs split into early-stage embryos, the chromosomes inside them are removed – the donor’s are replaced with chromosomes from the mother’s egg – the final egg will hopefully grow into a healthy embryo before implantation.