Inbreeding Video Lecture | Biology for Grade 12

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.