- [Instructor] We know that
any sexually reproducing organism
is getting DNA from both its male parent
and its female parent
and that's true also for human beings.
You might know we have 23 pairs of chromosomes
but let's zoom in on one of those pairs.
So let's say this is one of the chromosomes in a pair
and let's say that this is the other chromosome in the pair.
So one came from one parent.
The other came from the other parent.
Chromosomes are nothing more than bunched up strands of DNA
plus other things that help package it.
So let's unravel a little bit
to understand how these relate to genes.
And then, we'll dive
into a little bit more terminology around genetics.
So you have this big, long strand of DNA
and segments of this code for different genes.
Maybe this part right over here
is related to how tall you become.
So let's say that part right over there,
that might be related to hair color.
And in other videos,
we talk about how these genes
can essentially be translated eventually
into proteins which are expressed in forms
like height or hair color or eye color,
or many, many, many, many other ways.
But each of these chromosomes
actually code, generally speaking,
for the same genes.
So if this one has a region that codes for that gene,
this one will as well
and this one will also.
The other chromosome in the pair will also have a region
that codes for this other gene.
Now, as you can imagine,
the genes that get translated into the proteins
that eventually are show up in your hair color,
they can come in different versions.
So what we call the different versions of a gene
is an allele.
So for example, from this parent,
you might have gotten the allele here
that would describe,
let's say, blonde hair,
I'll denote that with lowercase b,
while from this parent,
you might have gotten the allele
that's associated with say, brown hair.
So they're both associated or they're both coding
for the same type of protein
that might play a role in hair color
but they're different versions of it.
So you could view an allele
as versions or a version,
I'll say, alleles are versions of a gene.
Now, if you know which versions you have,
then you're able to say, "I know my or I know
the organism's genotype."
So in this situation, we know we have the capital B allele
which is for the brown hair
and we have the lowercase b allele
which is for the blonde hair,
and so the genotype in this situation,
where you actually know what alleles you have,
you would say that that is, you have a capital B
from one parent
and you have the lowercase b
from another parent.
Now the thing you might be asking is,
but what does this organism look like?
How does this genotype actually get expressed?
And if you're asking that question
where you're asking a question of phenotype.
Phenotype is what is expressed
and this is usually observable in some way.
So the phenotype in this situation
might be, and it's seldom this simple in genetics,
but we're gonna make it simple
in this scenario,
that if you have a brown allele and blonde allele,
that maybe the brown allele is dominant
which is that if you have even one brown allele,
at the end of the day, you're going to see brown.
So I'll just write that down for this scenario
although that's not actually going to be the case.
Genetics gets a lot more complicated than that
but certain genes, certain alleles,
they are that simply dominant or recessive.
Now you can imagine there's many different genotypes
based on what I just described
that could result in this phenotype.
You could have this genotype that we just showed.
You could get a brown allele from both parents.
Either of these genotypes would express themselves
as the phenotype brown, if we assume,
and that is a big if,
if we assume that this is the dominant version of the gene
or the dominant allele.
If that is, then in either case,
it will express itself as the phenotype of brown.
The only way you would get blonde hair in this scenario
is if you had two of the recessive gene
or the recessive version of the gene
or the recessive allele.
This would lead to a blonde phenotype
right over here.
We've talked about dominant alleles.
We've talked about recessive alleles
and another piece of terminology
that you will hear when you talk about genetics
are notions of homozygous, homozygous
and/or versus heterozygous genotypes.
And when people are talking
about homozygous or heterozygous,
they're talking about the genotype.
Homozygous, you might know the prefix homo refers to same.
So this is a situation
where both the alleles are the same, same alleles,
and heterozygous is a situation where hetero,
that's referring to different, different alleles.
So what would we call this scenario right over here?
We would call this a heterozygous genotype for this gene.
This would be homozygous dominant.
We have two of the same dominant allele
and this would be homozygous recessive.
I'll leave you there.
You're now armed to talk a lot about genetics.