Meet Cosmo, The World's First Bull Designed To Produce Mostly Male Offspring

Cosmo was born at 110 pounds and healthy in April 2020. Alison Van Eenennaam/UC Davis

Jack Dunhill 28 Jul 2020, 11:47

Meet Cosmo. He may look like a normal calf, but he’s actually a breakthrough in genetic engineering. Cosmo has been genetically edited to produce mostly male offspring.

Scientists at the University of California, Davis used a genetic editing technology called CRISPR to insert a large gene that will make 75 percent of Cosmo’s offspring male, regardless of whether they inherit XX or XY chromosomes. Cosmo was born in April 2020, weighing in at 50 kilograms (110 pounds) and remains healthy. 

Presented at the American Society of Animal Science meeting on July 23, the scientists performed a "knock-in" – the insertion of a genetic sequence – on the calf embryo to add a male-determining gene. The team hope that the future offspring will inherit the gene, called the Sex-determining Region Y (Sry), and 75 percent will develop as male.

Sry is a gene that usually resides on the Y chromosome (hence the name) and initiates the development of male sexual characteristics, like the testes. Usually only half of all offspring inherit a Y chromosome and with it, an Sry gene.

To increase the chances of Cosmo’s offspring being male, the researchers aimed to insert the Sry gene into the X sex chromosome. This way, no matter what sex chromosome the offspring inherits, it could also inherit the new Sry and be male. However, the researchers tried to insert it into the X chromosome, which would mean 100 percent of Cosmo’s offspring would be male, but were unsuccessful, so they settled for chromosome 17. This means Cosmo’s offspring will be 50 percent XY (male), 25 percent XX but inherit the version of chromosome 17 with Sry and be male, and 25 percent XX with a normal chromosome 17 and be female.

After a difficult journey, the team was successful in inserting the gene into their target.

"It took two and a half years to develop the method to insert a gene into the developing embryo and another two years to successfully establish a pregnancy," said Joey Owen, one of the lead researchers, in a press release.

"We anticipate Cosmo's offspring that inherit this SRY gene will grow and look like males, regardless of whether they inherit a Y chromosome," added Alison Van Eenennaam, animal geneticist with the UC Davis Department of Animal Science.

Cosmo showing off his ear tags. Alison Van Eenennaam/UC Davis

Producing more male cattle means producing more beef. Eenennaam states that male cows are 15 percent better at turning feed into weight, more fuel-efficient, and tend to produce higher total weights than females. If more cattle can be born male, it’s possible that fewer animals will need to be slaughtered for beef.

Genetically engineered animals in the food industry is tightly controlled, so Cosmo and his offspring won’t be used for beef any time soon. Instead, he will be used to study how expressing Sry on chromosome 17 affects offspring, and whether they come out as normal, healthy males.

Either way, this is a massive leap forward in genetic editing and the food industry as a whole, and it’s interesting to see how modern editing technologies may shape the food supply of the future.



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