Incest, usually defined as sex between close blood relatives, is one of the most pervasive sexual taboos across cultures. Many different theories have been advanced to explain this taboo, but perhaps the most common is that we evolved to avoid incestuous relations because inbreeding increases the odds of health problems in any offspring produced.

So just how risky is incest anyway?

Our friends over at ASAP Science answer this question in the video below. As they explain, you first need to understand how a given trait comes about in the first place, including the concepts of dominant and recessive genes. In a nutshell, inbreeding increases the odds of rare genetic disorders because the more closely related two people are, the more likely it is that they’ll both be carriers of the same recessive genes, which gives these genes a chance of pairing up.

However, it’s important to note that even if both parents are carriers of disorder-linked recessive genes, odds are that their offspring will not develop the disorder. In other words, while inbreeding increases this risk, it’s not guaranteed. Check out the full video to learn more.