Being fatherless took on new meaning in April when a research team, led by Tomohiro Kono of the Tokyo University of Agriculture, created mice from two eggs. The group's achievement does not promise a new way to make babies; rather it helps to explain how egg and sperm work together and why males are vital in normal reproduction.

The process that created the mice is akin to parthenogenesis, in which an unfertilized egg develops on its own and produces viable offspring. It occurs in some lower creatures such as fleas, lizards and turkeys. The barrier to parthenogenesis in mammals is thought to be genetic imprinting, in which some genes needed for embryonic development are turned off in the female genome but switched on in the male genome, and vice versa. Thus, for an embryo to grow properly, it must have one set of chromosomes with a female imprint and the other with a male imprint. In past studies, mouse eggs have been induced to replicate without fertilization, but they survive only briefly.

This article was originally published with the title Mickey Has Two Moms.

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