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  • 0:04 What Is Inbreeding?
  • 0:54 Outbreeding
  • 2:33 Inbreeding
  • 4:01 Inbreeding on Purpose
  • 4:46 Lesson Summary
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Inbreeding: Definition and Effects

Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

You know you're not supposed to marry your sister or even your cousin. But why? In this lesson, learn why inbreeding can be harmful, but on the other hand, why it's sometimes done on purpose.

What Is Inbreeding?

When you were little, did you ever kind of fall in love with your mom or your dad, and maybe even say you wanted to marry them someday? And then they said to you, 'No, dear, you can't marry your father.' When you asked why, your mom probably said that first of all, dad was married to her, and second of all, you just don't marry people in your family.

Now that you're so much older and wiser, you know that the reason you can't marry people in your family is because if you had kids, that would be inbreeding. But what is inbreeding, anyway? Inbreeding is defined as mating between individuals that are genetically related. Inbreeding is the opposite of outbreeding, which is mating between individuals that are not genetically related. In this lesson, we'll learn why inbreeding can be harmful but also why it's sometimes done on purpose.


Before we talk about the effects of inbreeding, let's talk about what happens in outbreeding. Outbreeding is what most people do by marrying and having children with people that they are not related to. What this does is mix unrelated genetic backgrounds together, which is healthy for populations. Why is it healthy?

A lot of genetic disorders, such as hemophilia, cystic fibrosis and sickle cell anemia, are inherited in an autosomal recessive manner. That means that an individual will have this disorder if they are homozygous recessive; that is, have two copies of the recessive disease allele.

They won't have the disorder if they are heterozygous, meaning they only have one copy of the disease allele. And they definitely won't have the disorder if they are homozygous dominant, meaning they have two copies of the normal allele. So, in the case of these disorders, you want to be homozygous dominant or heterozygous but not homozygous recessive for the disease allele.

When there's a lot of outbreeding, there's a higher chance that individuals will be heterozygous. That's because they have genetic material that's pretty much randomly mixed from all of the possible alleles in the population. This random mixing increases the probability that offspring will inherit two different alleles, one from their mother and one from their father.

For example, if your father is tall, dark and handsome and your mother is short, blonde and has cute freckles, you can imagine that you would get one tall allele and one short allele, as well as one dark hair allele and one blonde allele, and so on. A nice mixture that could lead to a pretty interesting looking child!


In contrast to outbreeding, inbreeding increases the proportion of homozygotes in a population. If you look at this diagram of a horse family tree, you can see why.

Inbreeding increases the risk of a homozygous recessive offspring being produced
Horse breeding chart

Let's say that there is a grandparent horse that is heterozygous for a recessive disease allele. This horse is fine, because the disease doesn't show up unless a horse is homozygous for that recessive allele, right? Even if that heterozygous horse mates with a homozygous dominant horse that doesn't have the disease allele at all, about half of the offspring are going to be heterozygous for that disease allele. But remember, they're fine too, because you don't get the disease unless you are homozygous recessive.

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