Mitochondrial Replacement Therapy

The birth of the world's first "three-parent" baby contained the blended DNA of the biological mother and biological father mixed in with an unrelated female donor. The technique mitochondrial replacement therapy was used to allow the biological mother to bypass a segment in her DNA sequence that leads to babies born with Leigh syndrome, a neurological disorder that causes death in children before the age of 3. This is done by lifting the nucleus out of the original mother leaving behind the defective mitochondria that causes Leigh syndrome. the nucleus is then placed into a healthy egg from the donor mother. The hybrid egg with the original mother's nuclear genes and the donor mother's cytoplasm and mitochondria that is fertilized by the biological fathers’ sperm is then placed into the healthy mother to carry the fetus to term.

After the fetus was carried to term and delivered. The fetus currently carries between 2.36 and 9.23 percent of potentially defective DNA, according to sampling of urine, hair follicles and circumcised foreskin. It was discovered that not all of the defective mitochondria can be eliminated.

https://www.newscientist.com/article/2107219-exclusive-worlds-first-baby-born-with-new-3-parent-technique/
http://www.sciencealert.com/world-first-in-ukraine-as-three-parent-baby-born-to-an-infertile-couple