The technique replaces an egg's defective DNA with healthy genetic material from a female donor, to prevent children suffering debilitating conditions such as muscular dystrophy.It is controversial because it would result in babies having DNA from three people - and effectively, two mothers.Last year, Britain's parliament changed the law to allow the procedure, but the Human Fertilisation and Embryology Authority has called for further evidence on the technique's safety and efficacy before licensing the treatment.Yesterday, scientists at Newcastle University said they had succeeded in carrying out the technique on fertilised human embryos, which were shown to develop well.However, they warned that in some instances up to 4% of mutated DNA had carried over into the embryos and said they could not guarantee disease prevention. In one instance, mutated DNA that had carried over was found to increase over time, until it reached disease-causing levels.The new research was published in the journal, Nature...