In vitro fertilization is a major and often last ditch treatment when a woman cannot have a baby. In this treatment, an egg is removed from a woman and fertilized with sperm outside her body, under very carefully controlled conditions. The fertilized egg is then implanted into a woman's uterus, in the hopes that the fertilized egg will develop normally and a healthy baby will be born. This technique is often used when a mother has blocked fallopian tubes, pelvic adhesions or endometriosis or when a father has a low sperm count or sperm with low motility.
Since we're talking about fertilizing an egg without the necessity of sex, this process offers some nifty possibilities. A lesbian couple can use an egg from one of them with donated sperm to have a baby; a gay couple can use their sperm to fertilize an egg that will be carried to term by a surrogate mother, or a straight couple can use either or both of their eggs and sperm and enlist the help of a surrogate.
The proposed UK regulations would take things one step further and replace the mitochondria from the egg with the mitochondria from another woman. You may remember from your biology class in high school that mitochondria are tiny organelles found inside most of the cells in living things like animals and plants. Mitochondria produce the energy that cells need to live and do the things the cell is meant to do. Without properly functioning mitochondria, the cell will waste and die.
In humans, mitochondria are found inside every cell except for red blood cells, and malfunctioning mitochondria can cause neuromuscular disorders that may result in blindness, heart disease and death. The really neat thing about mitochondria is that they have their own DNA, and the mitochondrial DNA always comes from the egg. In other words, all of the mitochondria in your body have their own genetic code, which is different from your chromosomes, the genetic code which you will pass on to your children, and comes exclusively from your mother.
The IVF technique approved by the British government would take the key parts of the mother's egg, before or after fertilization, and swap them into the egg from a third-party donor with healthy mitochondria before implantation into the body of the woman who will carry the child to term. The child conceived from this method would carry the chromosomes of the mother and the father, and the mitochondria of someone else. This is pretty cool, because all of the important DNA that makes you you comes from the chromosomes of your parents; the DNA of your mitochondria doesn't matter.
Here are two helpful graphics produced by the BBC from its report. In the process illustrated below, the key parts of a fertilized egg are implanted into the fertilized egg from a donor: