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is Scientific American 60 Second Science. I'm Eileen Augenbrand.

Every thoroughbred horse alive today is descended from three stallions and a larger selection of mares in 17th and 18th century England.

Since then, no horses outside this lineage have been permitted to enter the bloodline.

That's a problem.

We analysed genetic diversity at thousands of genetic markers across the entire genome

in more than 10,000 thoroughbred horses from all of the major breeding

regions of the world. This is the largest set of horses that's been examined in this way to date.

Emilyne Hill, professor at University College Dublin and chief scientist with an ownership

stake at an equine science company called Plus Vital.

And what we found was that there has been a highly significant increase in in breeding in the population over the last 45 years.

And probably the greatest increase in inbreeding is seen in the last 10 or 15 years.

Thoroughbred horse production is tightly controlled.

Artificial insemination is not permitted,

which means that breeding stallions get moved around a lot for meetups with females.

These so-called shuttle stallions can mate with hundreds of mares per mating season.

And there are two mating seasons, one in the northern hemisphere and the other in the southern.

In our study, 97% of the horses traced back to a single horse called Northern Dancer.

Arguably the most successful stallion of the 20th century and his descendants

have been the dominant sirelines in Australia and Europe for the last 25 years.

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