We all have two parents, a mom and a dad, and they each contributed half of our genes. We all learned this in high school biology. It’s that meiosis thing, with reduction divisions making sperm and eggs with only half the normal number of genes, and then their union restoring the full number.
But that isn’t the whole story. Our cells have mitochondria, little energy factories, that have their own genes. Not very many genes, just a few. There are thirty seven mitochondrial genes, compared to about 23 thousand in the nucleus. But these mitochondrial genes have very important functions. Defects in these genes can cause a number of diseases, including mitochondrial myopathy (with muscle problems), Leber’s hereditary optic neuropathy, which can cause blindness, and Leigh’s disease, which results in degradation of motor skills and eventual death.
Interestingly, the mitochondrial genes are quite exceptional, since they are only inherited from the mother. I actually showed this as a graduate student, way back in the early seventies. I analyzed the mitochondrial DNAs from horses and donkeys, and their mixed progeny, mules (male donkey and female horse parents) and hinnies (male horse and female donkey parents). The results showed that the mitochondrial DNA just came from mom. This turns out to be true in all mammals.
Now suppose that mom has a severe mitochondrial gene mutation that she’d rather not pass to her children. Normal procreation would result in all of her children receiving her mitochondrial DNA, with the mutation. But there is a way to fix that. We can give the fertilized egg a cytoplasm transplant from a donor egg without the mitochondrial DNA mutations.
The simplest way to achieve this would be to transfer the nucleus of the fertilized egg to a new egg from a healthy mother. The resulting child would have three parents. Half of the nuclear genes would come from mom, and half from dad. And all of the mitochondrial genes would come from the second mother that donated the egg with the healthy mitochondria.
This is very different from nuclear transfer cloning, where an adult nucleus is placed into a surrogate fertilized egg. Cloning is very inefficient, usually succeeding for only about one percent of nuclear transfers. And even then there is question about the health of the progeny.
In contrast, the three parent scheme, with the nucleus of the fertilized egg moved to the donor egg with the nucleus removed, requires no fancy reprogramming to turn an adult nucleus into an embryonic one. It would work extremely well, with no obvious consequences, except the resulting child would have healthy mitochondria.
And what is wrong with that??