UK became the first country to have officially approved procedures to create “three-parent” babies.
What exactly is the "three parent child"?  is used to replace mother’s faulty Mitochondrial DNA with healthy Mitochondria from a donor woman during IVF process, thus the name- “three-parent” baby.(Mitochondria is a cell organel found inside a living cell..it is the power house of cell as it produces ATP out of the food we consume)..
2-The donor’s mitochondria contribute just 37 genes to the child, compared with more than 20,000 from the parents. That is a negligible amount and far less than one would gain from a blood transfusion or organ transplant.
3-No other characteristics in terms of intelligence, eye colour, hair colour, height etc. are changed.
About Mitochondrial Disease...
1-The mitochondria are organelles inside cells that are involved in releasing energy by producing adenosine triphosphate (ATP), the key energy currency that drives metabolism.
2-Mitochondria are inherited solely from the mother and this results into cases of babies been born with rare mitochondrial diseases if mother has the faulty mitchochondrial DNA.