Multicolored, Calico cats are the product of multiple fathers. Is it possible for a human child to have more than two parents? Researchers have discovered that it may be possible for children to have one father, but two mothers. This would make us less likely to inherit diseases, most of which come from the mother’s (mitochondrial) DNA.

A mother’s genetic contribution to her child comes in two separable parts. The largest is the 23 chromosomes in the nucleus of an unfertilized egg. The father also contributes 23 chromosomes in the sperm that fertilizes the egg.

But the mother ALSO contributes what is known as mitochondrial DNA. Mutations in this DNA–like mutations in regular DNA from both parents–can cause diseases, and diseases carried by the mitochondrial DNA are rare–but there are LOTS of them, meaning there is about one chance in 5,000 that a child will develop one of these inherited diseases.

Geneticist Shoukhrat Mitalipov collected eggs from the ovaries of women with mutated mitochondria and others from donors with healthy mitochondria. He removed the nuclei of both, then discarded those from the healthy cells. He transplanted the DNA from the diseased cells into the healthy cells, and then fertilized the result with sperm and allowed the fertilized eggs to start dividing, showing that it was possible to produce a zygote with three parents.

Nearly all of the experimental eggs survived the replacement of their nuclei, and three-quarters were successfully fertilized. However, just over half of the resulting zygotes displayed abnormalities. That compared with an abnormality rate of just an eighth in control zygotes grown from untransplanted, healthy eggs. However, when he tested his zygotes, he could find no trace of mutated mitochondrial DNA in them, so the purpose of the procedure–to see if he could eliminate the transfer of disease-causing DNA–seems to have been achieved.

"Test tube babies" are now pretty well accepted, but it we may have a long road ahead before we accept the idea of kids with three parents.

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