Mutations in mitochondrial DNA can cause severe diseases and fertility problems. Since mitochondria are only inherited from the mother, a mother with faulty mitochondrial DNA will pass this on to her children without exception. Techniques involving the use of a donor egg with healthy mitochondria were developed to avoid this. However, the fact that the resulting embryo contains genetic material from three different individuals raised many ethical questions. Also, the science behind mitochondrial transfer hasn’t been fully elucidated. Nonetheless, a scientific group from New York went ahead and applied the techniques, and last April, a three-parent baby was born.