1
00:00:00,000 --> 00:00:00,490


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00:00:00,490 --> 00:00:04,790
Well, before we even knew what
DNA was, much less how it was

3
00:00:04,790 --> 00:00:07,160
structured or it was replicated
or even before we

4
00:00:07,160 --> 00:00:11,020
could look in and see meiosis
happening in cells, we had the

5
00:00:11,020 --> 00:00:14,700
general sense that offspring
were the products of some

6
00:00:14,700 --> 00:00:16,480
traits that their parents had.

7
00:00:16,480 --> 00:00:22,170
That if I had a guy with blue
eyes-- let me say this is the

8
00:00:22,170 --> 00:00:27,430
blue-eyed guy right here --and
then if he were to marry a

9
00:00:27,430 --> 00:00:32,800
brown-eyed girl-- Let's say this
is the brown-eyed girl.

10
00:00:32,800 --> 00:00:36,410
Maybe make it a little
bit more like a girl.

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00:00:36,410 --> 00:00:39,450
If he were to marry the
brown-eyed girl there, that

12
00:00:39,450 --> 00:00:42,230
most of the time, or maybe in
all cases where we're dealing

13
00:00:42,230 --> 00:00:45,030
with the brown-eyed girl,
maybe their kids are

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00:00:45,030 --> 00:00:46,870
brown-eyed.

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00:00:46,870 --> 00:00:50,210
Let me do this so they have a
little brown-eyed baby here.

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00:00:50,210 --> 00:00:53,000


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00:00:53,000 --> 00:00:54,980
And this is just something--
I mean, there's obviously

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00:00:54,980 --> 00:00:57,420
thousands of generations of
human beings, and we've

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00:00:57,420 --> 00:00:58,180
observed this.

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00:00:58,180 --> 00:01:00,590
We've observed that kids look
like their parents, that they

21
00:01:00,590 --> 00:01:05,280
inherit some traits, and that
some traits seem to dominate

22
00:01:05,280 --> 00:01:06,140
other traits.

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00:01:06,140 --> 00:01:09,960
One example of that tends to
be a darker pigmentation in

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00:01:09,960 --> 00:01:11,570
maybe the hair or the eyes.

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00:01:11,570 --> 00:01:15,710
Even if the other parent has
light pigmentation, the darker

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00:01:15,710 --> 00:01:17,900
one seems to dominate, or
sometimes, it actually ends up

27
00:01:17,900 --> 00:01:20,420
being a mix, and we've seen
that all around us.

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00:01:20,420 --> 00:01:23,410
Now, this study of what gets
passed on and how it gets

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00:01:23,410 --> 00:01:26,590
passed on, it's much older than
the study of DNA, which

30
00:01:26,590 --> 00:01:29,840
was really kind of discovered
or became a big deal in the

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00:01:29,840 --> 00:01:31,110
middle of the 20th century.

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00:01:31,110 --> 00:01:32,820
This was studied a long time.

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00:01:32,820 --> 00:01:36,750
And kind of the father of
classical genetics and

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00:01:36,750 --> 00:01:38,455
heredity is Gregor Mendel.

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00:01:38,455 --> 00:01:41,720


36
00:01:41,720 --> 00:01:45,650
He was actually a monk, and he
would mess around with plants

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00:01:45,650 --> 00:01:48,795
and cross them and see which
traits got passed and which

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00:01:48,795 --> 00:01:51,230
traits didn't get passed and
tried to get an understanding

39
00:01:51,230 --> 00:01:55,650
of how traits are passed from
one generation to another.

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00:01:55,650 --> 00:02:02,090
So when we do this, when we
study this classical genetics,

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00:02:02,090 --> 00:02:05,230
I'm going to make a bunch of
simplifying assumptions

42
00:02:05,230 --> 00:02:08,080
because we know that most of
these don't hold for most of

43
00:02:08,080 --> 00:02:11,140
our genes, but it'll give us a
little bit of sense of how to

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00:02:11,140 --> 00:02:16,340
predict what might happen
in future generations.

45
00:02:16,340 --> 00:02:21,270
So the first simplifying
assumption I'll make is that

46
00:02:21,270 --> 00:02:24,840
some traits have kind of this
all or nothing property.

47
00:02:24,840 --> 00:02:26,690
And we know that a lot
of traits don't.

48
00:02:26,690 --> 00:02:28,690
Let's say that there are in
the world-- and this is a

49
00:02:28,690 --> 00:02:35,220
gross oversimplification --let's
say for eye color,

50
00:02:35,220 --> 00:02:38,520
let's say that there
are two alleles.

51
00:02:38,520 --> 00:02:40,390
Now remember what
an allele was.

52
00:02:40,390 --> 00:02:44,030
An allele is a specific
version of a gene.

53
00:02:44,030 --> 00:02:48,400
So let's say that you could
have blue eye color or you

54
00:02:48,400 --> 00:02:52,460
could have brown eye color.

55
00:02:52,460 --> 00:02:55,220
That we live in a universe where
someone could only have

56
00:02:55,220 --> 00:02:58,320
one of these two versions
of the eye color gene.

57
00:02:58,320 --> 00:03:01,230
We know that eye color is far
more complex than that, so

58
00:03:01,230 --> 00:03:02,970
this is just a simplification.

59
00:03:02,970 --> 00:03:04,340
And let me just make
up another one.

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00:03:04,340 --> 00:03:14,110
Let me say that, I don't know,
maybe for tooth size, that's a

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00:03:14,110 --> 00:03:17,590
trait you won't see in any
traditional biology textbook,

62
00:03:17,590 --> 00:03:23,470
and let's say that there's one
trait for big teeth and

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00:03:23,470 --> 00:03:28,330
there's another allele
for small teeth.

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00:03:28,330 --> 00:03:30,850
And I want to make very clear
this distinction between a

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00:03:30,850 --> 00:03:32,100
gene and an allele.

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00:03:32,100 --> 00:03:35,230


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00:03:35,230 --> 00:03:37,740
I talked about Gregor Mendel,
and he was doing this in the

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00:03:37,740 --> 00:03:41,890
1850s well before we knew what
DNA was or what even

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00:03:41,890 --> 00:03:48,590
chromosomes were and how DNA was
passed on, et cetera, but

70
00:03:48,590 --> 00:03:53,060
let's go into the microbiology
of it to understand the

71
00:03:53,060 --> 00:03:53,840
difference.

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00:03:53,840 --> 00:03:56,170
So I have a chromosome.

73
00:03:56,170 --> 00:03:59,710
Let's say on some chromosome--
let me pick

74
00:03:59,710 --> 00:04:00,900
some chromosome here.

75
00:04:00,900 --> 00:04:02,930
Let's say this is
some chromosome.

76
00:04:02,930 --> 00:04:04,590
Let's say I got that
from my dad.

77
00:04:04,590 --> 00:04:09,130
And on this chromosome, there's
some location here--

78
00:04:09,130 --> 00:04:11,550
we could call that the locus on
this chromosome where the

79
00:04:11,550 --> 00:04:15,330
eye color gene is --that's
the location of

80
00:04:15,330 --> 00:04:16,579
the eye color gene.

81
00:04:16,579 --> 00:04:19,279
Now, I have two chromosomes,
one from my father and one

82
00:04:19,279 --> 00:04:22,010
from my mother, so let's say
that this is the chromosome

83
00:04:22,010 --> 00:04:23,260
from my mother.

84
00:04:23,260 --> 00:04:26,020


85
00:04:26,020 --> 00:04:27,800
We know that when they're
normally in the cell, they

86
00:04:27,800 --> 00:04:30,180
aren't nice and neatly organized
like this in the

87
00:04:30,180 --> 00:04:32,860
chromosome, but this is just to
kind of show you the idea.

88
00:04:32,860 --> 00:04:35,690
Let's say these are homologous
chromosomes so they code for

89
00:04:35,690 --> 00:04:36,970
the same genes.

90
00:04:36,970 --> 00:04:41,460
So on this gene from my mother
on that same location or

91
00:04:41,460 --> 00:04:45,710
locus, there's also the
eye color gene.

92
00:04:45,710 --> 00:04:51,020
Now, I might have the same
version of the gene and I'm

93
00:04:51,020 --> 00:04:52,760
saying that there's only
two versions of

94
00:04:52,760 --> 00:04:54,220
this gene in the world.

95
00:04:54,220 --> 00:04:56,690
Now, if I have the same version
of the gene-- I'm

96
00:04:56,690 --> 00:04:58,510
going to make a little
shorthand notation.

97
00:04:58,510 --> 00:05:01,101
I'm going to write big B--
Actually, let me do

98
00:05:01,101 --> 00:05:02,010
it the other way.

99
00:05:02,010 --> 00:05:04,040
I'm going to write little b
for blue and I'm going to

100
00:05:04,040 --> 00:05:07,420
write big B for brown.

101
00:05:07,420 --> 00:05:11,210
There's a situation where this
could be a little b and this

102
00:05:11,210 --> 00:05:12,800
could be a big B.

103
00:05:12,800 --> 00:05:17,010
And then I could write that my
genotype-- I have the allele,

104
00:05:17,010 --> 00:05:20,320
I have one big B from my
mom and I have one

105
00:05:20,320 --> 00:05:24,440
small b from my dad.

106
00:05:24,440 --> 00:05:29,120
Each of these instances, or
ways that this gene is

107
00:05:29,120 --> 00:05:30,660
expressed, is an allele.

108
00:05:30,660 --> 00:05:40,360
So these are two different
alleles-- let me write that

109
00:05:40,360 --> 00:05:42,880
--or versions of
the same gene.

110
00:05:42,880 --> 00:05:46,340
And when I have two different
versions like this, one

111
00:05:46,340 --> 00:05:50,140
version from my mom, one version
from my dad, I'm

112
00:05:50,140 --> 00:05:53,480
called a heterozygote, or
sometimes it's called a

113
00:05:53,480 --> 00:05:54,730
heterozygous genotype.

114
00:05:54,730 --> 00:06:00,410


115
00:06:00,410 --> 00:06:05,000
And the genotype is the exact
version of the alleles I have.

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00:06:05,000 --> 00:06:08,170
Let's say I had the
lowercase b.

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00:06:08,170 --> 00:06:11,690
I had the blue-eyed gene
from both parents.

118
00:06:11,690 --> 00:06:16,040
So let's say that I was
lowercase b, lowercase b, then

119
00:06:16,040 --> 00:06:18,600
I would have two identical
alleles.

120
00:06:18,600 --> 00:06:21,840
Both of my parents gave me the
same version of the gene.

121
00:06:21,840 --> 00:06:29,930
And this case, this genotype
is homozygous, or this is a

122
00:06:29,930 --> 00:06:33,900
homozygous genotype, or I'm a
homozygote for this trait.

123
00:06:33,900 --> 00:06:36,400


124
00:06:36,400 --> 00:06:38,510
Now, you might say,
Sal, this is fine.

125
00:06:38,510 --> 00:06:43,400
These are the traits that you
have. I have a brown from

126
00:06:43,400 --> 00:06:47,550
maybe my mom and a
blue from my dad.

127
00:06:47,550 --> 00:06:50,850
In this case, I have a blue
from both my mom and dad.

128
00:06:50,850 --> 00:06:54,590
How do we know whether my eyes
are going to be brown or blue?

129
00:06:54,590 --> 00:06:56,900
And the reality is it's
very complex.

130
00:06:56,900 --> 00:06:58,410
It's a whole mixture
of things.

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00:06:58,410 --> 00:07:02,830
But Mendel, he studied
things that showed

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00:07:02,830 --> 00:07:04,080
what we'll call dominance.

133
00:07:04,080 --> 00:07:09,260


134
00:07:09,260 --> 00:07:12,930
And this is the idea that
one of these traits

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00:07:12,930 --> 00:07:14,090
dominates the other.

136
00:07:14,090 --> 00:07:17,160
So a lot of people originally
thought that eye color,

137
00:07:17,160 --> 00:07:20,490
especially blue eyes,
was always dominated

138
00:07:20,490 --> 00:07:21,500
by the other traits.

139
00:07:21,500 --> 00:07:23,140
We'll assume that here,
but that's a gross

140
00:07:23,140 --> 00:07:24,510
oversimplification.

141
00:07:24,510 --> 00:07:34,200
So let's say that brown
eyes are dominant

142
00:07:34,200 --> 00:07:36,130
and blue are recessive.

143
00:07:36,130 --> 00:07:39,320


144
00:07:39,320 --> 00:07:42,960
I wanted to do that in blue.

145
00:07:42,960 --> 00:07:49,560
Blue eyes are recessive.

146
00:07:49,560 --> 00:07:52,330
If this is the case, and this
is a-- As I've said

147
00:07:52,330 --> 00:07:55,730
repeatedly, this is a gross
oversimplification.

148
00:07:55,730 --> 00:08:00,670
But if that is the case, then
if I were to inherit this

149
00:08:00,670 --> 00:08:05,540
genotype, because brown eyes
are dominant-- remember, I

150
00:08:05,540 --> 00:08:12,400
said the big B here represents
brown eye and the lowercase b

151
00:08:12,400 --> 00:08:16,670
is recessive --all you're going
to see for the person

152
00:08:16,670 --> 00:08:19,450
with this genotype
is brown eyes.

153
00:08:19,450 --> 00:08:20,880
So let me do this here.

154
00:08:20,880 --> 00:08:21,710
Let me write this here.

155
00:08:21,710 --> 00:08:27,680
So genotype, and then I'll
write phenotype.

156
00:08:27,680 --> 00:08:31,030
Genotype is the actual versions
of the gene you have

157
00:08:31,030 --> 00:08:33,780
and then the phenotypes
are what's expressed

158
00:08:33,780 --> 00:08:35,030
or what do you see.

159
00:08:35,030 --> 00:08:39,690


160
00:08:39,690 --> 00:08:43,860
So if I get a brown-eyed gene
from my dad-- And I want to do

161
00:08:43,860 --> 00:08:46,310
it in a big-- I want
to do it in brown.

162
00:08:46,310 --> 00:08:49,670
Let me do it in brown so
you don't get confused.

163
00:08:49,670 --> 00:08:54,490
So if I've have a brown-eyed
gene from my dad and a

164
00:08:54,490 --> 00:09:05,130
blue-eyed gene from my mom,
because the brown eye is

165
00:09:05,130 --> 00:09:08,580
recessive, the brown-eyed allele
is recessive-- And I

166
00:09:08,580 --> 00:09:11,370
just said a brown-eyed gene, but
what I should say is the

167
00:09:11,370 --> 00:09:13,870
brown-eyed version of the
gene, which is the brown

168
00:09:13,870 --> 00:09:16,820
allele, or the blue-eyed version
of the gene from my

169
00:09:16,820 --> 00:09:18,890
mom, which is the blue allele.

170
00:09:18,890 --> 00:09:22,290
Since the brown allele is
dominant-- I wrote that up

171
00:09:22,290 --> 00:09:25,410
here --what's going to be
expressed are brown eyes.

172
00:09:25,410 --> 00:09:30,830


173
00:09:30,830 --> 00:09:34,450
Now, let's say I had
it the other way.

174
00:09:34,450 --> 00:09:39,850
Let's say I got a blue-eyed
allele from my dad and I get a

175
00:09:39,850 --> 00:09:41,750
brown-eyed allele for my mom.

176
00:09:41,750 --> 00:09:42,490
Same thing.

177
00:09:42,490 --> 00:09:46,940
The phenotype is going
to be brown eyes.

178
00:09:46,940 --> 00:09:49,730
Now, what if I get a brown-eyed
allele from both my

179
00:09:49,730 --> 00:09:52,470
mom and my dad?

180
00:09:52,470 --> 00:09:54,930
Let me see, I keep changing
the shade of brown, but

181
00:09:54,930 --> 00:09:55,960
they're all supposed
to be the same.

182
00:09:55,960 --> 00:09:59,130
So let's say I get two dominant
brown-eyed alleles

183
00:09:59,130 --> 00:10:00,820
from my mom and my dad.

184
00:10:00,820 --> 00:10:01,770
Then what are you
going to see?

185
00:10:01,770 --> 00:10:02,640
Well, you could guess that.

186
00:10:02,640 --> 00:10:08,280
I'm still going to
see brown eyes.

187
00:10:08,280 --> 00:10:10,570
So there's only one last
combination because these are

188
00:10:10,570 --> 00:10:12,800
the only two types of alleles
we might see in our

189
00:10:12,800 --> 00:10:15,510
population, although for
most genes, there's

190
00:10:15,510 --> 00:10:16,710
more than two types.

191
00:10:16,710 --> 00:10:18,400
For example, there's
blood types.

192
00:10:18,400 --> 00:10:21,490
There's four types of blood.

193
00:10:21,490 --> 00:10:25,540
But let's say that I get two
blue, one blue allele from

194
00:10:25,540 --> 00:10:30,400
each of my parents, one from
my dad, one from my mom.

195
00:10:30,400 --> 00:10:33,080
Then all of a sudden, this is a
recessive trait, but there's

196
00:10:33,080 --> 00:10:34,550
nothing to dominate it.

197
00:10:34,550 --> 00:10:39,130
So, all of a sudden, the
phenotype will be blue eyes.

198
00:10:39,130 --> 00:10:42,380
And I want to repeat again, this
isn't necessarily how the

199
00:10:42,380 --> 00:10:45,130
alleles for eye color work, but
it's a nice simplification

200
00:10:45,130 --> 00:10:48,370
to maybe understand how
heredity works.

201
00:10:48,370 --> 00:10:52,040
There are some traits that can
be studied in this simple way.

202
00:10:52,040 --> 00:10:54,920
But what I wanted to do here
is to show you that many

203
00:10:54,920 --> 00:10:58,970
different genotypes-- so these
are all different genotypes

204
00:10:58,970 --> 00:11:02,090
--they all coded for
the same phenotype.

205
00:11:02,090 --> 00:11:05,000
So just by looking at someone's
eye color, you

206
00:11:05,000 --> 00:11:09,000
didn't know exactly whether
they were homozygous

207
00:11:09,000 --> 00:11:16,740
dominant-- this would be
homozygous dominant --or

208
00:11:16,740 --> 00:11:19,080
whether they were
heterozygotes.

209
00:11:19,080 --> 00:11:21,350
This is heterozygous
right here.

210
00:11:21,350 --> 00:11:23,650
These two right here
are heterozygotes.

211
00:11:23,650 --> 00:11:27,990


212
00:11:27,990 --> 00:11:31,680
These are also sometimes called
hybrids, but the word

213
00:11:31,680 --> 00:11:33,600
hybrid is kind of overloaded.

214
00:11:33,600 --> 00:11:36,800
It's used a lot, but in this
context, it means that you got

215
00:11:36,800 --> 00:11:40,940
different versions of the
allele for that gene.

216
00:11:40,940 --> 00:11:43,740
So let's think a little bit
about what's actually

217
00:11:43,740 --> 00:11:48,155
happening when my mom and
my dad reproduced.

218
00:11:48,155 --> 00:11:50,970


219
00:11:50,970 --> 00:11:53,100
Well, let's think of a couple
of different scenarios.

220
00:11:53,100 --> 00:11:55,950


221
00:11:55,950 --> 00:11:57,630
Let's say that they're
both hybrids.

222
00:11:57,630 --> 00:12:03,470
My dad has the brown-eyed
dominant allele and he also

223
00:12:03,470 --> 00:12:08,020
has the blue-eyed recessive
allele.

224
00:12:08,020 --> 00:12:11,560
Let's say my mom has the same
thing, so brown-eyed dominant,

225
00:12:11,560 --> 00:12:14,780
and she also has the blue-eyed
recessive allele.

226
00:12:14,780 --> 00:12:17,880
Now let's think about if these
two people, before you see

227
00:12:17,880 --> 00:12:20,630
what my eye color is, if you
said, look, I'm giving you

228
00:12:20,630 --> 00:12:22,760
what these two people's
genotypes are.

229
00:12:22,760 --> 00:12:24,010
Let me label them.

230
00:12:24,010 --> 00:12:26,200


231
00:12:26,200 --> 00:12:27,790
Let me make this the mom.

232
00:12:27,790 --> 00:12:30,090
I think this is the standard
convention.

233
00:12:30,090 --> 00:12:34,730
And let's make this right
here, this is the dad.

234
00:12:34,730 --> 00:12:37,910
What are the different genotypes
that their children

235
00:12:37,910 --> 00:12:38,490
could have?

236
00:12:38,490 --> 00:12:40,630
So let's say they reproduce.

237
00:12:40,630 --> 00:12:44,090
I'm going to draw a
little grid here.

238
00:12:44,090 --> 00:12:45,660
So let me draw a grid.

239
00:12:45,660 --> 00:12:50,390


240
00:12:50,390 --> 00:12:55,990
So we know from our study of
meiosis that, look, my mom has

241
00:12:55,990 --> 00:12:59,870
this gene on-- Let me draw
the genes again.

242
00:12:59,870 --> 00:13:02,240
So there's a homologous
pair, right?

243
00:13:02,240 --> 00:13:04,880
This is one chromosome
right here.

244
00:13:04,880 --> 00:13:07,070
That's another chromosome
right there.

245
00:13:07,070 --> 00:13:10,130
On this chromosome in the
homologous pair, there might

246
00:13:10,130 --> 00:13:16,760
be-- at the eye color locus
--there's the brown-eyed gene.

247
00:13:16,760 --> 00:13:19,470
And at this one, at the eye
color locus, there's a

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00:13:19,470 --> 00:13:20,890
blue-eyed gene.

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00:13:20,890 --> 00:13:24,630
And similarly from my dad, when
you look at that same

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00:13:24,630 --> 00:13:28,310
chromosome in his cells-- Let
me do them like this.

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00:13:28,310 --> 00:13:30,740
So this is one chromosome there
and this is the other

252
00:13:30,740 --> 00:13:32,760
chromosome here.

253
00:13:32,760 --> 00:13:35,120
When you look at that locus
on this chromosome or that

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00:13:35,120 --> 00:13:37,870
location, it has the brown-eyed
allele for that

255
00:13:37,870 --> 00:13:40,370
gene, and on this one,
it has the blue-eyed

256
00:13:40,370 --> 00:13:41,590
allele on this gene.

257
00:13:41,590 --> 00:13:44,680
And we learn from meiosis when
the chromosomes-- Well, they

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00:13:44,680 --> 00:13:47,580
replicate first, and so you have
these two chromatids on a

259
00:13:47,580 --> 00:13:48,140
chromosome.

260
00:13:48,140 --> 00:13:51,520
But they line up in meiosis
I during the metaphase.

261
00:13:51,520 --> 00:13:53,220
And we don't know which
way they line up.

262
00:13:53,220 --> 00:13:56,510
For example, my dad might give
me this chromosome or might

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00:13:56,510 --> 00:13:57,630
give me that chromosome.

264
00:13:57,630 --> 00:13:59,790
Or my mom might give me that
chromosome or might give me

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00:13:59,790 --> 00:14:00,820
that chromosome.

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00:14:00,820 --> 00:14:02,760
So I could have any of
these combinations.

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00:14:02,760 --> 00:14:06,540
So, for example, if I get this
chromosome from my mom and

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00:14:06,540 --> 00:14:09,760
this chromosome from my dad,
what is the genotype going to

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00:14:09,760 --> 00:14:11,000
be for eye color?

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00:14:11,000 --> 00:14:16,770
Well, it's going to be capital
B and capital B.

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00:14:16,770 --> 00:14:21,510
If I get this chromosome from
my mom and this chromosome

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00:14:21,510 --> 00:14:22,620
from my dad, what's
it going to be?

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00:14:22,620 --> 00:14:28,330
Well, I'm going to get the big
B from my dad and then I'm

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00:14:28,330 --> 00:14:30,790
going to get the lowercase
b from my mom.

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00:14:30,790 --> 00:14:32,790
So this is another
possibility.

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00:14:32,790 --> 00:14:35,510
Now, this is another possibility
here where I get

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00:14:35,510 --> 00:14:42,490
the brown-eyed allele from my
mom and I get the blue eye

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00:14:42,490 --> 00:14:44,380
allele from my dad.

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00:14:44,380 --> 00:14:47,350
And then there's a possibility
that I get this chromosome

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00:14:47,350 --> 00:14:51,260
from my dad and this chromosome
from my mom, so

281
00:14:51,260 --> 00:14:53,520
it's this situation.

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00:14:53,520 --> 00:14:55,700
Now, what are the phenotypes
going to be?

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00:14:55,700 --> 00:14:58,290
Well, we've already seen that
this one right here is going

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00:14:58,290 --> 00:15:03,080
to be brown, that one's going to
be brown, this one's going

285
00:15:03,080 --> 00:15:06,250
to be brown, but this one
is going to be blue.

286
00:15:06,250 --> 00:15:07,860
I already showed you this.

287
00:15:07,860 --> 00:15:09,980
But if I were to tell you ahead
of time that, look, I

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00:15:09,980 --> 00:15:11,090
have two people.

289
00:15:11,090 --> 00:15:13,980
They're both hybrids, or they're
both heterozygotes for

290
00:15:13,980 --> 00:15:16,610
eye color, and eye
color has this

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00:15:16,610 --> 00:15:18,335
recessive dominant situation.

292
00:15:18,335 --> 00:15:22,530
And they're both heterozygotes
where they each have one brown

293
00:15:22,530 --> 00:15:24,980
allele and one blue allele, and
they're going to have a

294
00:15:24,980 --> 00:15:28,835
child, what's the probability
that the child has brown eyes?

295
00:15:28,835 --> 00:15:35,670


296
00:15:35,670 --> 00:15:37,170
What's the probability?

297
00:15:37,170 --> 00:15:40,720
Well, each of these scenarios
are equally likely, right?

298
00:15:40,720 --> 00:15:42,400
There's four equal scenarios.

299
00:15:42,400 --> 00:15:44,130
So let's put that in
the denominator.

300
00:15:44,130 --> 00:15:45,950
Four equal scenarios.

301
00:15:45,950 --> 00:15:48,110
And how many of those
scenarios end

302
00:15:48,110 --> 00:15:49,780
up with brown eyes?

303
00:15:49,780 --> 00:15:52,110
Well, it's one, two, three.

304
00:15:52,110 --> 00:15:58,780
So the probability is 3/4, or
it's a 75% probability.

305
00:15:58,780 --> 00:16:01,830
Same logic, what's the
probability that these parents

306
00:16:01,830 --> 00:16:04,650
produce an offspring
with blue eyes?

307
00:16:04,650 --> 00:16:07,280
Well, that's only one of
the four equally likely

308
00:16:07,280 --> 00:16:15,840
possibilities, so blue
eyes is only 25%.

309
00:16:15,840 --> 00:16:19,400
Now, what is the probability
that they produce a

310
00:16:19,400 --> 00:16:20,390
heterozygote?

311
00:16:20,390 --> 00:16:23,130
So what is the probability
that they produce a

312
00:16:23,130 --> 00:16:24,425
heterozygous offspring?

313
00:16:24,425 --> 00:16:27,360


314
00:16:27,360 --> 00:16:29,300
So now we're not looking at
the phenotype anymore.

315
00:16:29,300 --> 00:16:31,050
We're looking at the genotype.

316
00:16:31,050 --> 00:16:34,310
So of these combinations,
which are heterozygous?

317
00:16:34,310 --> 00:16:36,570
Well, this one is, because
it has a mix.

318
00:16:36,570 --> 00:16:37,360
It's a hybrid.

319
00:16:37,360 --> 00:16:39,380
It has a mix of the
two alleles.

320
00:16:39,380 --> 00:16:41,170
And so is this one.

321
00:16:41,170 --> 00:16:42,220
So what's the probability?

322
00:16:42,220 --> 00:16:45,050
Well, there's four different
combinations.

323
00:16:45,050 --> 00:16:48,020
All of those are equally likely,
and two of them result

324
00:16:48,020 --> 00:16:49,200
in a heterozygote.

325
00:16:49,200 --> 00:16:54,580
So it's 2/4 or 1/2 or 50%.

326
00:16:54,580 --> 00:16:56,570
So using this Punnett square,
and, of course, we had to make

327
00:16:56,570 --> 00:16:59,880
a lot of assumptions about the
genes and whether one's

328
00:16:59,880 --> 00:17:02,050
dominant or one's a recessive,
we can start to make

329
00:17:02,050 --> 00:17:03,880
predictions about the
probabilities

330
00:17:03,880 --> 00:17:05,530
of different outcomes.

331
00:17:05,530 --> 00:17:07,300
And as we'll see in future
videos, you can actually even

332
00:17:07,300 --> 00:17:07,970
go backwards.

333
00:17:07,970 --> 00:17:10,680
You can say, hey, given that
this couple had five kids with

334
00:17:10,680 --> 00:17:14,160
brown eyes, what's the
probability that they're both

335
00:17:14,160 --> 00:17:15,819
heterozygotes, or something
like that.

336
00:17:15,819 --> 00:17:19,000
So it's a really interesting
area, even though it is a bit

337
00:17:19,000 --> 00:17:20,490
of oversimplification.

338
00:17:20,490 --> 00:17:23,760
But many traits, especially some
of the things that Gregor

339
00:17:23,760 --> 00:17:27,190
Mendel studied, can be
studied in this way.