Reengineering Life

Scientists Gene Edited a Cow to Have Mostly Male Offspring

The technique could eventually lead to fewer cattle needed to produce the same amount of beef

Emily Mullin
Published in
5 min readJul 28, 2020
Photo illustration; Image source: Alison Van Eenennaam/UC Davis

Reengineering Life is a series from OneZero about the astonishing ways genetic technology is changing humanity and the world around us.

On a sunny Tuesday in April, amid a global pandemic, a newborn calf took his first shaky steps in a barn outside Sacramento. Animal geneticist Alison Van Eenennaam and postdoctoral researcher Joey Owen, looked on in awe. This wasn’t just any bull calf. This animal had been gene edited so that he could eventually produce more male offspring than normal.

The bull calf, affectionately named Cosmo, was the result of five years of research. Nine months earlier, Van Eenennaam, Owen, and a team of seven other scientists used the gene-editing tool CRISPR to insert a gene into cow embryos, including the embryo that eventually became Cosmo. The gene, SRY, is responsible for male sex development in cattle, and adding it to a female essentially swaps its sex. The researchers’ goal was to make a bull that would father mostly boys.

“The idea was actually suggested to me by a cattle operation,” Van Eenennaam, PhD, a researcher at the University of California, Davis, tells OneZero.

Male cattle are better for beef production than their female counterparts because they have bigger muscles, gain weight faster, and require less food to put on extra pounds. Overall, bulls are about 15% more efficient than females at converting food into weight, she says. More males could mean fewer cattle to produce the same amount of beef — a win for ranchers and potentially the environment.

To make Cosmo, the researchers injected about 200 cow embryos with CRISPR and the SRY gene. They also added a green fluorescent protein, commonly used in biomedical research, that would glow if an embryo was successfully edited. About two dozen embryos ended up surviving in the lab, and just nine of those embryos glowed green.

CRISPR works by making a targeted break in DNA using a guide molecule that finds the right location in the genome and a cutting protein that slices the…



Emily Mullin

Former staff writer at Medium, where I covered biotech, genetics, and Covid-19 for OneZero, Future Human, Elemental, and the Coronavirus Blog.