The UK House of Commons voted today to allow scientists to combine the DNA of three people instead of two. The law must now be approved by the House of Lords, the UK's upper house, reports CNN. If the law passes in the upper house, it may change ethical standards around genetic engineering.

The vote was intended to allow women with mitochondrial disease conceive healthy babies. There are two kinds of DNA: nuclear, for which a person can thank both parents, and mitochondrial, which comes only from the mother. The technique for mitochondrial replacement uses DNA from three people. A woman donates an egg, and scientists replace her nuclear DNA — responsible for traits like height and hair color — with that of the prospective mother, while leaving the donor's mitochondrial DNA intact. Because mitochondria are located outside the nucleus, the resulting egg contains only healthy mitochondrial DNA — 37 genes belonging to the donor — as well as 25,000 genes from the prospective mother. The egg is then fertilized by the father. DNA from all three people would remain with the child for life.

Mitochondria are the tiny, energy-producing "batteries" inside the cell, and are passed down only through the mother, unlike nuclear DNA, which draws from both parents. Mitochondrial disease occurs when the body's storehouses don't create enough energy, and is considered rare. So if a mother has mitochondrial disease, her child will have it too. About 6,500 children are born with faulty mitochondria worldwide each year. It can cause numerous health problems, including seizures, blindness, heart problems, and liver disease. Given these complications and the fact mitochondrial DNA is only passed on by women, potential mothers who suffer from mitochondrial disease often end up adopting or forgoing having children for fear of passing on a life-threatening disease.

"Mitochondrial disease is unimaginably cruel," mitochondrial disease advocacy groups wrote in an open letter to UK lawmakers this week, reports Reuters. "It strips our children of the skills they have learned, inflicts pain that cannot be managed and tires their organs one by one until their little bodies cannot go on any more."

Children have already been born using a slightly different technique that injected donor mitochondrial DNA into eggs after fertilization — almost 30, from 1997 until 2003. The US FDA cracked down on the process in 2003, telling fertility clinics that genetically manipulated embryos are subject to regulation as a biological product.

Some scientists worry that allowing the technique to take place would effectively legalize the creation of children that are genetically designed to avoid certain traits. "This would be the first case of intentionally manipulating a child's genome," David King, director of Human Genetic Alert, a UK-based public watchdog group, told The Verge back in June. "And we are crossing that line for the sake of what exactly? For a very small number of families who already have options that allow them to raise children."

Once the law is approved, clinics will be able to submit an application to the Human Fertilization and Embryology Authority, which can decide if it's the right thing to do and if the proposal satisfies the ethical criteria, Kent says. "[The law] basically gives the HFEA the power to authorize mitochondrial replacement."

UK Prime Minister David Cameron has spoken out in favor of the procedure. "I think it has be thoroughly researched and tested, and as someone who had a severely disabled child myself, I know what parents go through when they are concerned about these issues," Cameron has said, reports the BBC. "So science can help in this way, all the arguments are in favor, we should make sure these treatments are available."

The vote means that the UK is very close to allowing mitochondrial replacement to occur. As soon as the law is approved, the UK's Human Fertilization and Embryology Authority will be able to start to giving out licenses for the procedure, which means the first birth could take place in 2016.

Correction: An earlier version of this article stated that the law had been fully approved by the UK government. That is incorrect. The law was approved by the House of Commons, but still needs to go through the House of Lords. The article has been changed to reflect the law's status.