Mendel started his assumptions with some specific
rules. You will recall that he looked at some
very distinct traits that exhbited certain
outcomes and he did this on purpose so that
he could find some general rule for how inheritance
patterns happened. Now lots of things exhbit
Mendelian inheritance patterns, but don't
necessarily have the same sorts of phenotypes.
Mendel had determined that each trait was
specified by a single gene, whether it was
flower color or seed shape or whatever. He
by chance perhaps, but picked that one single
gene would specify that trait and that there
were only two alternatives he only saw round
or wrinkled. He only saw tall or short and
the gene products acted independently at each
other, so there was no dependence of one gene
like in a metabolic pathway. You could imagine
if you have enzyme A and its broken, you don't
get the product B and thus you wouldn't get
C. So independence of the genes and then also
he didn't choose any genes that had environmental
effects or he did not chose traits because
he didn't know about genes.
Let us look at some examples where Mendelian
inheritance is happening, but not getting
the expected phenotypes. Skin color is a great
example of a continuously variant trait, so
is height. Right there isn't black or white.
There isn't tall or short. There are many
different varieties on a continuum. Generally
when we see these sorts of things, it involves
multiple genes. Here is a very simplistic
example of how that might work out. Let us
say there were three genes for skin color.
Skin color comes about by the production of
pigment and there are three genes involved
in the production of that pigment. If we were
to use a Punnett square to predict the outcomes
of this, it would be quite complicated, but
you can see that there is a continous spectrum
of color that could be obtained dependent
on how many alleles are displayed for pigment.
This is a really simplified view of how it works,
but it does demonstrate the idea of multiple
genes or having a continous effect. Polygenic
inheritance is meaning that we have multiple
genes involved in the expression of this trait.
Height is another example and we can see here
that height in general form a continous or
bell shaped curve with less very small people,
lots of medium people and less very tall people.
It is on a continuum any time you see a continuum
we generally are talking about polygenic inheritance
or multiple genes being involved.
As we move on, we can see that other genes
have more than one effect. For example, we
have a single gene and it ends up in the production
of one phenotype A or B or C. Examples of
this can be found in all sorts of different
situations, but let us look at one where one
phenotype might actually be a lethal phenotype.
Here we are looking at Agouti mice and in
this case, we have an allele for yellow. The
Y is dominant for yellow color, but it is
lethal when there are two copies for some
reason. In this case, when we cross two heterozygotes,
we end up with one recessive trait, which
would be the brown Agouti mouse and the two
yellows because the y allele is dominant.
But in the bottom corner, we see two dominant
y alleles that are lethal and if the mouse
has two copies of it, then it does not make
it to live and so the ratio here is skewed.
We have a 1 to 2 ratio in the offspring. Consider
again that we have a number of offsprings
not just four, but we're counting probably
in the 10s and 100s to see these ratios.
Another example where Mendel's ratio didn't
play out and we may have this pleiotrophic
inheritance is in albinism and sickle cell
anemia. In albinism, someone is lacking the
enzyme to produce pigment and that has multiple
different effects not only do they not have
pigment in their hair, but also in eyelashes
and in skin and a number of different
phenotypes are associated with one gene mutation.
Sickle cell anemia is a great example of pleiotrophic
inheritance. It results from our hemoglobin
molecule having a mutation in the beta subunits
and so when it stacks up inside the red blood
cell not only does it not carry oxygen, but
it causes a sickling of a cell, which is the
secondary effect and that sickled cell will
tend to get stuck in some of the smaller blood
vessels. There are a number of circulatory
phenotypes associated with just one mutation.
Later we will get to understand that it is
just one tiny base change in the DNA that
ends up causing these multiple or pleiotrophic