change an animal’s sex

(Illustration: Frida Holme)

From glow-in-the-dark mice, less farty cows to silk-spinning goats, genetic modification of animals is nothing new. CRISPR has even been used successfully to create cold resistant pigs and TB resistant cows. Getting approval for genetically modified animals to enter the US food chain is, however, more of a novelty. The FDA has approved a genetically modified salmon, but the approval took decades and has been met with intense opposition from environmental and food-safety groups.

In one of the Obama Administration’s last acts, the U.S. Food and Drug Administration said it intended to treat CRISPR-edited animals as if they were new drugs, requiring elaborate and costly safety studies.  This stymied hopes for a lighter touch for gene edited livestock animals.

The Daily Bundle

An article by MIT Technology Review describes project “Boys Only,” in which Alison Van Eenennaam of the University of California aims to create a bull that will father only male offspring: either normal bull calves or ones with two X chromosomes but also the male-making SRY. No females at all.

The lab is using CRISPR to add a gene called SRY to some bovine skin cells. The presence of SRY can make a female turn out to be essentially male—with bigger muscles, a penis, and testicles.  Crucially the offspring are supposedly unable to make sperm and therefore spread the genetic modification further than intended.  

Cattle breeders already have a way to make only males. Because a sperm cell bearing a Y chromosome has slightly less DNA than its X counterpart and is thus lighter, bull semen containing these cells can be separated by weight. All-male semen is sold in catalogues.

But terminator bulls could be a better solution. Artificial insemination is used in only about 4 percent of beef cattle because of the effort involved in roving the range, gathering cows, and getting them pregnant. Van Eenennaam thinks her male-only bulls would be a less expensive way.