and how it's not really the best for a population.

So before we do that, let's review

the concept of natural selection.

And natural selection is the idea

that a member of a population that

has a special genetic trait that's advantageous

is more likely to live to an age where they can reproduce

and pass on that special trait to their offspring.

And you should also remember that a population can

get a lot out of having a big gene pool.

And the bigger the gene pool, the more genetic diversity

the population has, which allows the group

to adapt to many different environmental changes.

So what is inbreeding, exactly?

Well, inbreeding is when people in a population

will selectively have offspring with a certain smaller

group within that larger population.

And this can be for a bunch of reasons,

like religion or culture, or maybe just because

of preference.

And when inbreeding occurs with non-human populations,

it's almost always due to geographical barriers, where

the greater population simply isn't accessible.

Now, when people usually think of inbreeding,

words like "incest" come to mind.

But inbreeding really isn't limited to members

of the same family having offspring together.

Lots of small religious and cultural groups in the world

have some people with common ancestors

and are only distantly related.

So you can see that the effects of inbreeding

can exist without close relatives actually having

children together.

So why is inbreeding a problem in the first place?

Well, let's look at an example.

So Tay-Sachs disease is an autosomal recessive disorder.

And what that means is that people

with no copies of the genes are unaffected by the disease.

And I've drawn these people in blue.

People with just one copy of the gene

are not affected by the disease, but are carriers for the gene.

And I've drawn these people in red.

And people with two copies of the gene

are affected by the disease.

And I've drawn these guys in purple.

So let's say we have someone who's a carrier for Tay-Sachs.

So he has just one copy of the gene.

If we're looking at the general population,

we can see that the odds of the person choosing a mate that's

also a carrier for the disease are pretty low.

And if he eventually has some kids, none of them

will be affected by the disease, and only a few

will even be carriers.

It's likely that the copies of the gene

will be so spread out among the population

that it would be quite rare for two carriers

to actually end up mating together.

Now if we look at an inbred population

where a bunch more people could be carriers for the disease,

the chances of our guy choosing a mate that's also a carrier

are a little higher.

So more of his children will be carriers for the disease.

But there's also a chance that some of his offspring

may get two copies of the gene and actually

be affected by the disease.

Now, we just talked about an example

with an autosomal recessive disorder.

But maybe you're wondering how inbreeding

affects autosomal dominant disorders.

Well, let's look at Huntington's disease, which

is autosomal dominant.

And since this disease is autosomal dominant,

if a person has no copies of the gene,

they'll be unaffected by the gene.

And I've drawn these people in blue once again.

However, if a person has either one or two copies of the gene,

then that person will be affected

by the disease either way.

And I've drawn both of these people in red.

The key difference in this case is that

no matter who the guy has children with,

even if that guy just has one copy of the Huntington's gene,

there's still a chance that there

will be children affected by the disease.

Now, of course, if our guy has children with someone

who was also affected by the disease,

then more of his children would be affected.

But there's still a chance either way.

Now, one of the other reasons why we're less concerned

about inbreeding affecting autosomal dominant diseases

is that carriers for dominant disorders

are generally aware that they're affected and are well

aware of the risks of them having diseased children.

With recessive disorders, carriers usually

don't have any symptoms at all.

And they may not even know that they're

carriers until they've had a diseased child.

And this makes it much more important

for people in inbred populations to seek genetic counseling so

that they are aware of the risks of them

having diseased children.

So what did we learn?

Well, first we learned that certain inbred populations

can have many more individuals that

may carry a diseased chromosome than the general population.

But we also learned that this is mostly

a concern with autosomal recessive diseases,

since those generally go more unnoticed

than dominant ones do.