Mitochondria provide the energy for cells, but unlike DNA, they are only inherited from our mothers through their eggs. Defective mitochondria cause diseases which are incurable in adulthood.

However, these diseases are preventable before conception with a treatment called “cytoplasmic transfer,” where the nucleus of a human egg is transferred to a “donor egg” of a different woman with healthy mitochondria. Before the transfer, the DNA from the donor egg is removed. This means the embryo still has the DNA from both parents, but mitochondria from the donor.

Though this technique is ‘old hat’ in research terms, as it first shown as successful over a decade ago, the practice was discontinued in the US due to ethical concerns over genetic engineering of humans.

Some discussions can be found here:
CNN: http://www.cnn.com/2014/02/26/health/ivf-mitochondria/
NYTimes: http://well.blogs.nytimes.com/2013/12/16/three-biological-parents-and-a-baby/
PLOS: http://blogs.plos.org/dnascience/2014/03/06/mitohype-3-parent-designer-babies-revisited/

Technical articles can be found here:
http://www.sciencedirect.com/science/article/pii/S0015028213032901
http://www.ncbi.nlm.nih.gov/pubmed/22085724

Edited by Stephanie Hays from the Harvard Systems Biology.