Genetics: Choosing the Sex of Rabbits

In the Middle Ages, women who wanted a boy baby were advised to avoid copulation during the dark of the moon and (while an abbot prayed) to drink wine, mixed by an alchemist, with lion’s blood. As often as not, they gave birth to girls. And despite scientists’ growing understanding of genetics, modern parents are unable to do any better in choosing the sex of their offspring. But help may be on the way. Two English scientists have devised a technique for controlling the sex of rabbits. Their method, the first to achieve 100% accuracy with any mammal, may some day be applied to humans.

Cambridge University Physiologists Richard Gardner and Robert Edwards reported in Nature that they mated rabbits, then from the females took fertilized egg cells that had already grown into tiny embryos but had not yet become implanted in the uterine wall. They placed each embryo under a microscope, cut a tiny slit in its surrounding membrane and drew out several hundred cells with a suction pipette. The cells were then examined for the presence of sex chromatin, a substance found only in female cells. Separated into male and female groups, the embryos were next placed in a culture medium, a laboratory equivalent of a hospital recovery room.

After several hours, the fully recovered and sex-identified embryos were dropped into slits made in the uterus of mature female rabbits that had been treated with hormones to make the uterine walls receptive to implantation of the embryos. In each of 18 completed pregnancies, the female rabbits gave birth to young of the sex predetermined by the scientists.

The next application of the new technique, Physiologist Edwards believes, will be with such animals as sheep and cows. Most mammals can be induced to produce extra eggs, he says, by hormone treatments. Thus an impregnated cow could produce as many as four embryos that could be flushed out, sex-identified and selectively reimplanted. Since milk-producing cows are far more valuable than a plethora of bulls, the practice promises economic advantages. Human sex determination will be far more difficult, the scientists caution. Obtaining human eggs, fertilizing them on the laboratory bench and culturing the early embryos to the point where sex identification is possible are techniques still far beyond today’s laboratory skills. “We may get to human embryos,” says Edwards, “but that will be in the long, long future.”