For thousands of years, we humans have been breeding dogs for certain physical and behavioral traits. But along with the good, we’ve also gotten the bad.

Over the past two centuries, much of the focus has been on breeding for physical — make that cosmetic — traits. And line breeding, which is breeding of direct relatives, is a reliable way to get there. Unfortunately, that process also increases the risk of passing along genetic weaknesses.

Just look at the English bulldog, whose desired body type makes natural breeding and birthing impossible — and breathing nearly so. Likewise, numerous inherited traits are passed down other breed lines too.

If you’re set on a purebred dog, you can take steps to look for genetic glitches. Ask the breeder for results of medical tests and a bloodline pedigree of diseases before you commit.