-[Voiceover] An introduction to Mendelian Genetics.
Now before we start, let's review the idea
that human cells contain 46 chromosomes,
which contain the DNA that makes each cell unique.
23 of these chromosomes were inherited
from a person's father
and 23 were inherited from the mother.
We can say that each person's made up
of a combination of genetic code
from both of their parents.
Now sometimes we like to say
that we have 23 pairs of chromosomes.
Instead of saying that we have 46 total
because that way we remind ourselves
that for each chromosome we have a maternal
and paternal copy.
Now the first thing I want to introduce
is the term allele.
If we have a chromosome here
and then an allele is one small section
on that chromosome that codes for a specific gene
that makes you, you.
Since humans have at least two copies
of each chromosome,
we can say that humans usually have
at least two alleles for every specific gene.
One allele from their mother and one from their father.
Let's look at an example
and we'll start by talking about blood type.
I'm sure that you've heard that blood types
are usually named with letters
like A, B, and O.
What does that actually mean?
Well there's a specific allele
that codes for blood type.
Let's say that we have this guy here
and his alleles both code for blood type A.
I'll use the letter A for that.
Let's say we have this girl here
who has one allele coding for A
and another allele coding for blood type O.
Now for the guy, he has both alleles
coding for blood type A
then it's pretty clear that when we check
his actual blood type it will be A.
For the girl, we're not so sure
since she has one of each.
Now, I'm going to introduce a couple new terms to you.
The first is that since the guy has two alleles
that code with the same thing
both code for blood type A
then we say that this guy is homozygous.
Homo means the two alleles are the same,
homo the same
and zygous refers to mixture of DNA
that he got from his parents.
Someone who is homozygous got the same allele
from both parents.
In the case of the girl,
is she going to have blood type A or blood type O?
Well it turns out that she's going to have blood type A
and that's because the A allele is the dominant allele.
While the O allele is the recessive allele.
When an allele is dominant that means
if someone has two different alleles
it will be the dominant one that wins.
In this case since A is dominant over O
which is recessive,
A will win and she'll have blood type A.
Since this girl has two different alleles
we call her heterozygous
since hetero means different
and zygous refers to the same thing,
a mixture of DNA that she got from her parents.
Now I want to introduce two more terms.
We can describe a person's genes in two different ways.
We can look at the person's individual alleles
and we call this the genotype.
For this guy his genotype is AA
referring to his two alleles
which both code for blood type A.
We can also look at a person's physical traits
which we call the phenotype.
For this guy and girl the phenotype would be
blood type A.
You can see that genotype and phenotype are different
but it is possible for two different genotypes
to make the same phenotype.
Since some alleles are dominant over others.
Let's talk about gene inheritance for a bit.
Let's say that our guy and girl from before
have offspring together.
We can use something called a Punnett Square
to determine what different genotypes
their kids could have.
Each of the parents two alleles
are on separate chromosomes,
so each parent will contribute
one of their two alleles to the child.
The Punnett Square allows you to determine
all possible combinations.
If we take the father's alleles
and line them up vertically
and then take the mother's alleles
and line them up horizontally,
we can fill in the chart to find the possible genotypes
for our offspring.
In this case, two of our boxes will have the AA in them
and two will have AO in them.
That means half of the children
will have the genotype AA
and half of the children will have genotype AO.
Since both of these genotypes code for the same phenotype
all of the children will have the blood type A phenotype.
Let's see what happens if we change our father's genotype
to match our mother's genotype.
Now only one-quarter of the children
will have the AA genotype,
half will have the AO genotype
since the order of the two alleles doesn't matter
OA and AO are the same.
One quarter will have the OO genotype.
This means that 75% of the children
will have blood type A in their phenotype.
Since AA and AO make blood type A
but 25% of the children
will have the blood type O phenotype,
since OO makes blood type O.
What did we learn?
Well first we learned what an allele is
and the difference between homozygous
and heterozygous,
as well as the difference between
dominant and recessive traits in relation to alleles.
Second, we learned about the difference
between genotype and phenotype
and how the genotype refers to a persons DNA
while a phenotype refers to the physical traits
that the DNA codes for.
Finally we learned about how we can use
a Punnett Square to determine
how different alleles will be inherited
from two parents.