Hello today we're going to be talking
about epistasis.
Epistasis is basically the expression of
different genes that influence a
phenotype,
so in this example I have two
chromosomes with
different alleles expressed at different
loci, so one over here
and one over here. And then for this
chromosome
we have one phenotype being expressed
and over at this chromosome we have a
different phenotype being expressed.
What epistasis deals with is if both of
these
chromosomes, interact so both loci
interact, and then they
eventually produce either the first
phenotype
or the second phenotype
and this basically happens because
one of these alleles, whether it's this
one or
this one, has a property that allows them
to mask the other alleles. So basically
epistasis is an interaction of two
different alleles that produce one
phenotype. So the epistasis that i'm
going to be talking about today
is duplicate recessive epistasis.
Basically if you have double
a or double b recessive alleles or both
then you're going to end up with
the recessive phenotype being expressed.
So essentially if you want the dominant
phenotype to be expressed then you need
both loci to have a dominant allele.
In this pathway here I'm going to be
using the example from Avatar: The Last
Airbender where we have
people who can waterbend and people who
can firebend. I'm not going to focus on earthbending and airbending.
But in this example we're going to say
that waterbending is recessive,
so just going to say recessive, and then
we're going to say
firebending is dominant.
And so in this pathway for epistasis to
work,
if we want to cross over here we would
need the
dominant A allele to be activated.
However if we have
both recessive then this pathway is not
going to work,
it'll just stay as a waterbender. But
then,
we're still at the waterbending
phenotype and to get to firebending we
need
the B dominant allele to also be
activated
and if we have only recessive, then again
we're just stuck at being waterbenders,
so these two phenotypes are exactly the
same,
because they both carry recessive
alleles, but if we have
both dominant alleles over here
then we'll end up as a firebender.
Using duplicate recessive epistasis if
we did a dihybrid cross between two
heterozygous individuals
we would have nine firebenders due to
the dominant alleles.
And then we would have seven waterbenders
due to there being at least two
recessive alleles, so ratio will be nine
to seven. And then if we did another
dihybrid cross over here,
I already did the math, we would have a
ratio of one to three
due to this having at least two dominant
alleles so it would be one
which would be fire, and then over here
we have recessive
so this would be waterbenders.
