0:00:00.000,0:00:00.490


0:00:00.490,0:00:04.790
Well, before we even knew what[br]DNA was, much less how it was

0:00:04.790,0:00:07.160
structured or it was replicated[br]or even before we

0:00:07.160,0:00:11.020
could look in and see meiosis[br]happening in cells, we had the

0:00:11.020,0:00:14.700
general sense that offspring[br]were the products of some

0:00:14.700,0:00:16.480
traits that their parents had.

0:00:16.480,0:00:22.170
That if I had a guy with blue[br]eyes-- let me say this is the

0:00:22.170,0:00:27.430
blue-eyed guy right here --and[br]then if he were to marry a

0:00:27.430,0:00:32.800
brown-eyed girl-- Let's say this[br]is the brown-eyed girl.

0:00:32.800,0:00:36.410
Maybe make it a little[br]bit more like a girl.

0:00:36.410,0:00:39.450
If he were to marry the[br]brown-eyed girl there, that

0:00:39.450,0:00:42.230
most of the time, or maybe in[br]all cases where we're dealing

0:00:42.230,0:00:45.030
with the brown-eyed girl,[br]maybe their kids are

0:00:45.030,0:00:46.870
brown-eyed.

0:00:46.870,0:00:50.210
Let me do this so they have a[br]little brown-eyed baby here.

0:00:50.210,0:00:53.000


0:00:53.000,0:00:54.980
And this is just something--[br]I mean, there's obviously

0:00:54.980,0:00:57.420
thousands of generations of[br]human beings, and we've

0:00:57.420,0:00:58.180
observed this.

0:00:58.180,0:01:00.590
We've observed that kids look[br]like their parents, that they

0:01:00.590,0:01:05.280
inherit some traits, and that[br]some traits seem to dominate

0:01:05.280,0:01:06.140
other traits.

0:01:06.140,0:01:09.960
One example of that tends to[br]be a darker pigmentation in

0:01:09.960,0:01:11.570
maybe the hair or the eyes.

0:01:11.570,0:01:15.710
Even if the other parent has[br]light pigmentation, the darker

0:01:15.710,0:01:17.900
one seems to dominate, or[br]sometimes, it actually ends up

0:01:17.900,0:01:20.420
being a mix, and we've seen[br]that all around us.

0:01:20.420,0:01:23.410
Now, this study of what gets[br]passed on and how it gets

0:01:23.410,0:01:26.590
passed on, it's much older than[br]the study of DNA, which

0:01:26.590,0:01:29.840
was really kind of discovered[br]or became a big deal in the

0:01:29.840,0:01:31.110
middle of the 20th century.

0:01:31.110,0:01:32.820
This was studied a long time.

0:01:32.820,0:01:36.750
And kind of the father of[br]classical genetics and

0:01:36.750,0:01:38.455
heredity is Gregor Mendel.

0:01:38.455,0:01:41.720


0:01:41.720,0:01:45.650
He was actually a monk, and he[br]would mess around with plants

0:01:45.650,0:01:48.795
and cross them and see which[br]traits got passed and which

0:01:48.795,0:01:51.230
traits didn't get passed and[br]tried to get an understanding

0:01:51.230,0:01:55.650
of how traits are passed from[br]one generation to another.

0:01:55.650,0:02:02.090
So when we do this, when we[br]study this classical genetics,

0:02:02.090,0:02:05.230
I'm going to make a bunch of[br]simplifying assumptions

0:02:05.230,0:02:08.080
because we know that most of[br]these don't hold for most of

0:02:08.080,0:02:11.140
our genes, but it'll give us a[br]little bit of sense of how to

0:02:11.140,0:02:16.340
predict what might happen[br]in future generations.

0:02:16.340,0:02:21.270
So the first simplifying[br]assumption I'll make is that

0:02:21.270,0:02:24.840
some traits have kind of this[br]all or nothing property.

0:02:24.840,0:02:26.690
And we know that a lot[br]of traits don't.

0:02:26.690,0:02:28.690
Let's say that there are in[br]the world-- and this is a

0:02:28.690,0:02:35.220
gross oversimplification --let's[br]say for eye color,

0:02:35.220,0:02:38.520
let's say that there[br]are two alleles.

0:02:38.520,0:02:40.390
Now remember what[br]an allele was.

0:02:40.390,0:02:44.030
An allele is a specific[br]version of a gene.

0:02:44.030,0:02:48.400
So let's say that you could[br]have blue eye color or you

0:02:48.400,0:02:52.460
could have brown eye color.

0:02:52.460,0:02:55.220
That we live in a universe where[br]someone could only have

0:02:55.220,0:02:58.320
one of these two versions[br]of the eye color gene.

0:02:58.320,0:03:01.230
We know that eye color is far[br]more complex than that, so

0:03:01.230,0:03:02.970
this is just a simplification.

0:03:02.970,0:03:04.340
And let me just make[br]up another one.

0:03:04.340,0:03:14.110
Let me say that, I don't know,[br]maybe for tooth size, that's a

0:03:14.110,0:03:17.590
trait you won't see in any[br]traditional biology textbook,

0:03:17.590,0:03:23.470
and let's say that there's one[br]trait for big teeth and

0:03:23.470,0:03:28.330
there's another allele[br]for small teeth.

0:03:28.330,0:03:30.850
And I want to make very clear[br]this distinction between a

0:03:30.850,0:03:32.100
gene and an allele.

0:03:32.100,0:03:35.230


0:03:35.230,0:03:37.740
I talked about Gregor Mendel,[br]and he was doing this in the

0:03:37.740,0:03:41.890
1850s well before we knew what[br]DNA was or what even

0:03:41.890,0:03:48.590
chromosomes were and how DNA was[br]passed on, et cetera, but

0:03:48.590,0:03:53.060
let's go into the microbiology[br]of it to understand the

0:03:53.060,0:03:53.840
difference.

0:03:53.840,0:03:56.170
So I have a chromosome.

0:03:56.170,0:03:59.710
Let's say on some chromosome--[br]let me pick

0:03:59.710,0:04:00.900
some chromosome here.

0:04:00.900,0:04:02.930
Let's say this is[br]some chromosome.

0:04:02.930,0:04:04.590
Let's say I got that[br]from my dad.

0:04:04.590,0:04:09.130
And on this chromosome, there's[br]some location here--

0:04:09.130,0:04:11.550
we could call that the locus on[br]this chromosome where the

0:04:11.550,0:04:15.330
eye color gene is --that's[br]the location of

0:04:15.330,0:04:16.579
the eye color gene.

0:04:16.579,0:04:19.279
Now, I have two chromosomes,[br]one from my father and one

0:04:19.279,0:04:22.010
from my mother, so let's say[br]that this is the chromosome

0:04:22.010,0:04:23.260
from my mother.

0:04:23.260,0:04:26.020


0:04:26.020,0:04:27.800
We know that when they're[br]normally in the cell, they

0:04:27.800,0:04:30.180
aren't nice and neatly organized[br]like this in the

0:04:30.180,0:04:32.860
chromosome, but this is just to[br]kind of show you the idea.

0:04:32.860,0:04:35.690
Let's say these are homologous[br]chromosomes so they code for

0:04:35.690,0:04:36.970
the same genes.

0:04:36.970,0:04:41.460
So on this gene from my mother[br]on that same location or

0:04:41.460,0:04:45.710
locus, there's also the[br]eye color gene.

0:04:45.710,0:04:51.020
Now, I might have the same[br]version of the gene and I'm

0:04:51.020,0:04:52.760
saying that there's only[br]two versions of

0:04:52.760,0:04:54.220
this gene in the world.

0:04:54.220,0:04:56.690
Now, if I have the same version[br]of the gene-- I'm

0:04:56.690,0:04:58.510
going to make a little[br]shorthand notation.

0:04:58.510,0:05:01.101
I'm going to write big B--[br]Actually, let me do

0:05:01.101,0:05:02.010
it the other way.

0:05:02.010,0:05:04.040
I'm going to write little b[br]for blue and I'm going to

0:05:04.040,0:05:07.420
write big B for brown.

0:05:07.420,0:05:11.210
There's a situation where this[br]could be a little b and this

0:05:11.210,0:05:12.800
could be a big B.

0:05:12.800,0:05:17.010
And then I could write that my[br]genotype-- I have the allele,

0:05:17.010,0:05:20.320
I have one big B from my[br]mom and I have one

0:05:20.320,0:05:24.440
small b from my dad.

0:05:24.440,0:05:29.120
Each of these instances, or[br]ways that this gene is

0:05:29.120,0:05:30.660
expressed, is an allele.

0:05:30.660,0:05:40.360
So these are two different[br]alleles-- let me write that

0:05:40.360,0:05:42.880
--or versions of[br]the same gene.

0:05:42.880,0:05:46.340
And when I have two different[br]versions like this, one

0:05:46.340,0:05:50.140
version from my mom, one version[br]from my dad, I'm

0:05:50.140,0:05:53.480
called a heterozygote, or[br]sometimes it's called a

0:05:53.480,0:05:54.730
heterozygous genotype.

0:05:54.730,0:06:00.410


0:06:00.410,0:06:05.000
And the genotype is the exact[br]version of the alleles I have.

0:06:05.000,0:06:08.170
Let's say I had the[br]lowercase b.

0:06:08.170,0:06:11.690
I had the blue-eyed gene[br]from both parents.

0:06:11.690,0:06:16.040
So let's say that I was[br]lowercase b, lowercase b, then

0:06:16.040,0:06:18.600
I would have two identical[br]alleles.

0:06:18.600,0:06:21.840
Both of my parents gave me the[br]same version of the gene.

0:06:21.840,0:06:29.930
And this case, this genotype[br]is homozygous, or this is a

0:06:29.930,0:06:33.900
homozygous genotype, or I'm a[br]homozygote for this trait.

0:06:33.900,0:06:36.400


0:06:36.400,0:06:38.510
Now, you might say,[br]Sal, this is fine.

0:06:38.510,0:06:43.400
These are the traits that you[br]have. I have a brown from

0:06:43.400,0:06:47.550
maybe my mom and a[br]blue from my dad.

0:06:47.550,0:06:50.850
In this case, I have a blue[br]from both my mom and dad.

0:06:50.850,0:06:54.590
How do we know whether my eyes[br]are going to be brown or blue?

0:06:54.590,0:06:56.900
And the reality is it's[br]very complex.

0:06:56.900,0:06:58.410
It's a whole mixture[br]of things.

0:06:58.410,0:07:02.830
But Mendel, he studied[br]things that showed

0:07:02.830,0:07:04.080
what we'll call dominance.

0:07:04.080,0:07:09.260


0:07:09.260,0:07:12.930
And this is the idea that[br]one of these traits

0:07:12.930,0:07:14.090
dominates the other.

0:07:14.090,0:07:17.160
So a lot of people originally[br]thought that eye color,

0:07:17.160,0:07:20.490
especially blue eyes,[br]was always dominated

0:07:20.490,0:07:21.500
by the other traits.

0:07:21.500,0:07:23.140
We'll assume that here,[br]but that's a gross

0:07:23.140,0:07:24.510
oversimplification.

0:07:24.510,0:07:34.200
So let's say that brown[br]eyes are dominant

0:07:34.200,0:07:36.130
and blue are recessive.

0:07:36.130,0:07:39.320


0:07:39.320,0:07:42.960
I wanted to do that in blue.

0:07:42.960,0:07:49.560
Blue eyes are recessive.

0:07:49.560,0:07:52.330
If this is the case, and this[br]is a-- As I've said

0:07:52.330,0:07:55.730
repeatedly, this is a gross[br]oversimplification.

0:07:55.730,0:08:00.670
But if that is the case, then[br]if I were to inherit this

0:08:00.670,0:08:05.540
genotype, because brown eyes[br]are dominant-- remember, I

0:08:05.540,0:08:12.400
said the big B here represents[br]brown eye and the lowercase b

0:08:12.400,0:08:16.670
is recessive --all you're going[br]to see for the person

0:08:16.670,0:08:19.450
with this genotype[br]is brown eyes.

0:08:19.450,0:08:20.880
So let me do this here.

0:08:20.880,0:08:21.710
Let me write this here.

0:08:21.710,0:08:27.680
So genotype, and then I'll[br]write phenotype.

0:08:27.680,0:08:31.030
Genotype is the actual versions[br]of the gene you have

0:08:31.030,0:08:33.780
and then the phenotypes[br]are what's expressed

0:08:33.780,0:08:35.030
or what do you see.

0:08:35.030,0:08:39.690


0:08:39.690,0:08:43.860
So if I get a brown-eyed gene[br]from my dad-- And I want to do

0:08:43.860,0:08:46.310
it in a big-- I want[br]to do it in brown.

0:08:46.310,0:08:49.670
Let me do it in brown so[br]you don't get confused.

0:08:49.670,0:08:54.490
So if I've have a brown-eyed[br]gene from my dad and a

0:08:54.490,0:09:05.130
blue-eyed gene from my mom,[br]because the brown eye is

0:09:05.130,0:09:08.580
recessive, the brown-eyed allele[br]is recessive-- And I

0:09:08.580,0:09:11.370
just said a brown-eyed gene, but[br]what I should say is the

0:09:11.370,0:09:13.870
brown-eyed version of the[br]gene, which is the brown

0:09:13.870,0:09:16.820
allele, or the blue-eyed version[br]of the gene from my

0:09:16.820,0:09:18.890
mom, which is the blue allele.

0:09:18.890,0:09:22.290
Since the brown allele is[br]dominant-- I wrote that up

0:09:22.290,0:09:25.410
here --what's going to be[br]expressed are brown eyes.

0:09:25.410,0:09:30.830


0:09:30.830,0:09:34.450
Now, let's say I had[br]it the other way.

0:09:34.450,0:09:39.850
Let's say I got a blue-eyed[br]allele from my dad and I get a

0:09:39.850,0:09:41.750
brown-eyed allele for my mom.

0:09:41.750,0:09:42.490
Same thing.

0:09:42.490,0:09:46.940
The phenotype is going[br]to be brown eyes.

0:09:46.940,0:09:49.730
Now, what if I get a brown-eyed[br]allele from both my

0:09:49.730,0:09:52.470
mom and my dad?

0:09:52.470,0:09:54.930
Let me see, I keep changing[br]the shade of brown, but

0:09:54.930,0:09:55.960
they're all supposed[br]to be the same.

0:09:55.960,0:09:59.130
So let's say I get two dominant[br]brown-eyed alleles

0:09:59.130,0:10:00.820
from my mom and my dad.

0:10:00.820,0:10:01.770
Then what are you[br]going to see?

0:10:01.770,0:10:02.640
Well, you could guess that.

0:10:02.640,0:10:08.280
I'm still going to[br]see brown eyes.

0:10:08.280,0:10:10.570
So there's only one last[br]combination because these are

0:10:10.570,0:10:12.800
the only two types of alleles[br]we might see in our

0:10:12.800,0:10:15.510
population, although for[br]most genes, there's

0:10:15.510,0:10:16.710
more than two types.

0:10:16.710,0:10:18.400
For example, there's[br]blood types.

0:10:18.400,0:10:21.490
There's four types of blood.

0:10:21.490,0:10:25.540
But let's say that I get two[br]blue, one blue allele from

0:10:25.540,0:10:30.400
each of my parents, one from[br]my dad, one from my mom.

0:10:30.400,0:10:33.080
Then all of a sudden, this is a[br]recessive trait, but there's

0:10:33.080,0:10:34.550
nothing to dominate it.

0:10:34.550,0:10:39.130
So, all of a sudden, the[br]phenotype will be blue eyes.

0:10:39.130,0:10:42.380
And I want to repeat again, this[br]isn't necessarily how the

0:10:42.380,0:10:45.130
alleles for eye color work, but[br]it's a nice simplification

0:10:45.130,0:10:48.370
to maybe understand how[br]heredity works.

0:10:48.370,0:10:52.040
There are some traits that can[br]be studied in this simple way.

0:10:52.040,0:10:54.920
But what I wanted to do here[br]is to show you that many

0:10:54.920,0:10:58.970
different genotypes-- so these[br]are all different genotypes

0:10:58.970,0:11:02.090
--they all coded for[br]the same phenotype.

0:11:02.090,0:11:05.000
So just by looking at someone's[br]eye color, you

0:11:05.000,0:11:09.000
didn't know exactly whether[br]they were homozygous

0:11:09.000,0:11:16.740
dominant-- this would be[br]homozygous dominant --or

0:11:16.740,0:11:19.080
whether they were[br]heterozygotes.

0:11:19.080,0:11:21.350
This is heterozygous[br]right here.

0:11:21.350,0:11:23.650
These two right here[br]are heterozygotes.

0:11:23.650,0:11:27.990


0:11:27.990,0:11:31.680
These are also sometimes called[br]hybrids, but the word

0:11:31.680,0:11:33.600
hybrid is kind of overloaded.

0:11:33.600,0:11:36.800
It's used a lot, but in this[br]context, it means that you got

0:11:36.800,0:11:40.940
different versions of the[br]allele for that gene.

0:11:40.940,0:11:43.740
So let's think a little bit[br]about what's actually

0:11:43.740,0:11:48.155
happening when my mom and[br]my dad reproduced.

0:11:48.155,0:11:50.970


0:11:50.970,0:11:53.100
Well, let's think of a couple[br]of different scenarios.

0:11:53.100,0:11:55.950


0:11:55.950,0:11:57.630
Let's say that they're[br]both hybrids.

0:11:57.630,0:12:03.470
My dad has the brown-eyed[br]dominant allele and he also

0:12:03.470,0:12:08.020
has the blue-eyed recessive[br]allele.

0:12:08.020,0:12:11.560
Let's say my mom has the same[br]thing, so brown-eyed dominant,

0:12:11.560,0:12:14.780
and she also has the blue-eyed[br]recessive allele.

0:12:14.780,0:12:17.880
Now let's think about if these[br]two people, before you see

0:12:17.880,0:12:20.630
what my eye color is, if you[br]said, look, I'm giving you

0:12:20.630,0:12:22.760
what these two people's[br]genotypes are.

0:12:22.760,0:12:24.010
Let me label them.

0:12:24.010,0:12:26.200


0:12:26.200,0:12:27.790
Let me make this the mom.

0:12:27.790,0:12:30.090
I think this is the standard[br]convention.

0:12:30.090,0:12:34.730
And let's make this right[br]here, this is the dad.

0:12:34.730,0:12:37.910
What are the different genotypes[br]that their children

0:12:37.910,0:12:38.490
could have?

0:12:38.490,0:12:40.630
So let's say they reproduce.

0:12:40.630,0:12:44.090
I'm going to draw a[br]little grid here.

0:12:44.090,0:12:45.660
So let me draw a grid.

0:12:45.660,0:12:50.390


0:12:50.390,0:12:55.990
So we know from our study of[br]meiosis that, look, my mom has

0:12:55.990,0:12:59.870
this gene on-- Let me draw[br]the genes again.

0:12:59.870,0:13:02.240
So there's a homologous[br]pair, right?

0:13:02.240,0:13:04.880
This is one chromosome[br]right here.

0:13:04.880,0:13:07.070
That's another chromosome[br]right there.

0:13:07.070,0:13:10.130
On this chromosome in the[br]homologous pair, there might

0:13:10.130,0:13:16.760
be-- at the eye color locus[br]--there's the brown-eyed gene.

0:13:16.760,0:13:19.470
And at this one, at the eye[br]color locus, there's a

0:13:19.470,0:13:20.890
blue-eyed gene.

0:13:20.890,0:13:24.630
And similarly from my dad, when[br]you look at that same

0:13:24.630,0:13:28.310
chromosome in his cells-- Let[br]me do them like this.

0:13:28.310,0:13:30.740
So this is one chromosome there[br]and this is the other

0:13:30.740,0:13:32.760
chromosome here.

0:13:32.760,0:13:35.120
When you look at that locus[br]on this chromosome or that

0:13:35.120,0:13:37.870
location, it has the brown-eyed[br]allele for that

0:13:37.870,0:13:40.370
gene, and on this one,[br]it has the blue-eyed

0:13:40.370,0:13:41.590
allele on this gene.

0:13:41.590,0:13:44.680
And we learn from meiosis when[br]the chromosomes-- Well, they

0:13:44.680,0:13:47.580
replicate first, and so you have[br]these two chromatids on a

0:13:47.580,0:13:48.140
chromosome.

0:13:48.140,0:13:51.520
But they line up in meiosis[br]I during the metaphase.

0:13:51.520,0:13:53.220
And we don't know which[br]way they line up.

0:13:53.220,0:13:56.510
For example, my dad might give[br]me this chromosome or might

0:13:56.510,0:13:57.630
give me that chromosome.

0:13:57.630,0:13:59.790
Or my mom might give me that[br]chromosome or might give me

0:13:59.790,0:14:00.820
that chromosome.

0:14:00.820,0:14:02.760
So I could have any of[br]these combinations.

0:14:02.760,0:14:06.540
So, for example, if I get this[br]chromosome from my mom and

0:14:06.540,0:14:09.760
this chromosome from my dad,[br]what is the genotype going to

0:14:09.760,0:14:11.000
be for eye color?

0:14:11.000,0:14:16.770
Well, it's going to be capital[br]B and capital B.

0:14:16.770,0:14:21.510
If I get this chromosome from[br]my mom and this chromosome

0:14:21.510,0:14:22.620
from my dad, what's[br]it going to be?

0:14:22.620,0:14:28.330
Well, I'm going to get the big[br]B from my dad and then I'm

0:14:28.330,0:14:30.790
going to get the lowercase[br]b from my mom.

0:14:30.790,0:14:32.790
So this is another[br]possibility.

0:14:32.790,0:14:35.510
Now, this is another possibility[br]here where I get

0:14:35.510,0:14:42.490
the brown-eyed allele from my[br]mom and I get the blue eye

0:14:42.490,0:14:44.380
allele from my dad.

0:14:44.380,0:14:47.350
And then there's a possibility[br]that I get this chromosome

0:14:47.350,0:14:51.260
from my dad and this chromosome[br]from my mom, so

0:14:51.260,0:14:53.520
it's this situation.

0:14:53.520,0:14:55.700
Now, what are the phenotypes[br]going to be?

0:14:55.700,0:14:58.290
Well, we've already seen that[br]this one right here is going

0:14:58.290,0:15:03.080
to be brown, that one's going to[br]be brown, this one's going

0:15:03.080,0:15:06.250
to be brown, but this one[br]is going to be blue.

0:15:06.250,0:15:07.860
I already showed you this.

0:15:07.860,0:15:09.980
But if I were to tell you ahead[br]of time that, look, I

0:15:09.980,0:15:11.090
have two people.

0:15:11.090,0:15:13.980
They're both hybrids, or they're[br]both heterozygotes for

0:15:13.980,0:15:16.610
eye color, and eye[br]color has this

0:15:16.610,0:15:18.335
recessive dominant situation.

0:15:18.335,0:15:22.530
And they're both heterozygotes[br]where they each have one brown

0:15:22.530,0:15:24.980
allele and one blue allele, and[br]they're going to have a

0:15:24.980,0:15:28.835
child, what's the probability[br]that the child has brown eyes?

0:15:28.835,0:15:35.670


0:15:35.670,0:15:37.170
What's the probability?

0:15:37.170,0:15:40.720
Well, each of these scenarios[br]are equally likely, right?

0:15:40.720,0:15:42.400
There's four equal scenarios.

0:15:42.400,0:15:44.130
So let's put that in[br]the denominator.

0:15:44.130,0:15:45.950
Four equal scenarios.

0:15:45.950,0:15:48.110
And how many of those[br]scenarios end

0:15:48.110,0:15:49.780
up with brown eyes?

0:15:49.780,0:15:52.110
Well, it's one, two, three.

0:15:52.110,0:15:58.780
So the probability is 3/4, or[br]it's a 75% probability.

0:15:58.780,0:16:01.830
Same logic, what's the[br]probability that these parents

0:16:01.830,0:16:04.650
produce an offspring[br]with blue eyes?

0:16:04.650,0:16:07.280
Well, that's only one of[br]the four equally likely

0:16:07.280,0:16:15.840
possibilities, so blue[br]eyes is only 25%.

0:16:15.840,0:16:19.400
Now, what is the probability[br]that they produce a

0:16:19.400,0:16:20.390
heterozygote?

0:16:20.390,0:16:23.130
So what is the probability[br]that they produce a

0:16:23.130,0:16:24.425
heterozygous offspring?

0:16:24.425,0:16:27.360


0:16:27.360,0:16:29.300
So now we're not looking at[br]the phenotype anymore.

0:16:29.300,0:16:31.050
We're looking at the genotype.

0:16:31.050,0:16:34.310
So of these combinations,[br]which are heterozygous?

0:16:34.310,0:16:36.570
Well, this one is, because[br]it has a mix.

0:16:36.570,0:16:37.360
It's a hybrid.

0:16:37.360,0:16:39.380
It has a mix of the[br]two alleles.

0:16:39.380,0:16:41.170
And so is this one.

0:16:41.170,0:16:42.220
So what's the probability?

0:16:42.220,0:16:45.050
Well, there's four different[br]combinations.

0:16:45.050,0:16:48.020
All of those are equally likely,[br]and two of them result

0:16:48.020,0:16:49.200
in a heterozygote.

0:16:49.200,0:16:54.580
So it's 2/4 or 1/2 or 50%.

0:16:54.580,0:16:56.570
So using this Punnett square,[br]and, of course, we had to make

0:16:56.570,0:16:59.880
a lot of assumptions about the[br]genes and whether one's

0:16:59.880,0:17:02.050
dominant or one's a recessive,[br]we can start to make

0:17:02.050,0:17:03.880
predictions about the[br]probabilities

0:17:03.880,0:17:05.530
of different outcomes.

0:17:05.530,0:17:07.300
And as we'll see in future[br]videos, you can actually even

0:17:07.300,0:17:07.970
go backwards.

0:17:07.970,0:17:10.680
You can say, hey, given that[br]this couple had five kids with

0:17:10.680,0:17:14.160
brown eyes, what's the[br]probability that they're both

0:17:14.160,0:17:15.819
heterozygotes, or something[br]like that.

0:17:15.819,0:17:19.000
So it's a really interesting[br]area, even though it is a bit

0:17:19.000,0:17:20.490
of oversimplification.

0:17:20.490,0:17:23.760
But many traits, especially some[br]of the things that Gregor

0:17:23.760,0:17:27.190
Mendel studied, can be[br]studied in this way.