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.