So today, I want to talk to you about the effects of inbreeding
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.