Fish, insects, crustaceans, and even some plants possess the ability to change the sex of their offspring before they are born.

It's a genetic skill mammals lack. Or did.

A new study reveals a genetic system in mammals that enables two animals to mate and produce only females. A similar system based on identical principles would produce only males.

This proof of concept is in mice - an instance where a mouse study is actually valid - but the real benefit is for agriculture, where farmers may want dairy cows and egg-laying chickens and not have to deal with random chance. 

The researchers crossed two types of genetically engineered mice. The maternal mouse encoded a Cas9 protein, a CRISPR-protein that is inactive unless guided by special "guide-RNAs." The paternal mouse encoded these guide-RNAs on the Y-sex chromosome, a sex chromosome present only in males. After fertilization, the guide-RNAs from the paternal sperm and the Cas9 protein from the maternal egg were combined in the male mouse embryos, but not in the female embryos (because the females lack the Y chromosome). The combination of guide-RNAs with Cas9 results in a complex that eliminates the male embryos.

"We showed that Cas9 was specifically activated only in male embryos," says co-author Prof. Udi Qimron of Tel Aviv University. "Our results pave the way for a genetic system that allows biased sex production. When two transgenic types of mice encoding Cas9 or Y-chromosome-encoded guide-RNAs are crossed, lethality of males occurs because Cas9 is guided from the Y chromosome to target essential genes. This does not happen in females because the Y chromosome is not transferred to them. This cross thus halts embryonic development of males without affecting the development of females.

"Importantly, the system can also be used to produce only males. Engineering the guide-RNAs on the paternal X-sex chromosome should result in the exclusive elimination of females, resulting in males-only progeny, which are more beneficial in the beef industry."