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
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.
The problem happens when two heterozygous horses, like the brother and sister in this family tree, mate. Remember from basic genetics that their offspring have a one in four chance of being homozygous recessive and thus getting the disease. See why inbreeding increases the proportion of homozygotes?
You can compare it to outbreeding, which is also shown in this diagram. If a heterozygous horse is outbred, it's very unlikely that its mate will also be heterozygous for the same disease allele. Most likely, that horse from outside the family is normal and homozygous dominant for that particular allele. If that's the case, there's zero chance that their offspring will be homozygous recessive and inherit the disease. Again, this is why we say that outbreeding is healthier for a population.
Inbreeding on Purpose
Keep in mind, though, that sometimes inbreeding is actually done on purpose! This happens a lot in agriculture and animal breeding when a desirable trait is caused by being homozygous for a particular allele. For example, maybe the juiciest tomatoes come from being homozygous dominant for a certain allele. Or maybe the fastest racing dogs come from being homozygous. In these cases, inbreeding can help produce more homozygous offspring and thus more offspring that have that desirable trait.
However, it can also lead to undesirable effects. This is why, for example, pugs often have breathing problems. They were originally inbred to produce their extreme cuteness, but their population also has higher rates of certain genetic disorders than if they had been outbred.
In this lesson, we've learned more than we ever wanted to know about inbreeding. We learned that 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.
Then we learned about the effects of inbreeding. Inbreeding increases the proportion of homozygotes in a population. This can be harmful because inbreeding increases the chances of producing homozygous recessive individuals that have particular genetic disorders. However, inbreeding is sometimes done on purpose in agriculture or animal breeding. This is because being homozygous for certain genes can cause desirable traits, too.
Once you have finished this lesson you should be able to:
- Compare outbreeding and inbreeding
- Describe the benefits of outbreeding for a population
- Discuss the reasons inbreeding can be detrimental for a population
- Explain why inbreeding might be used in a target population