WEBVTT 00:00:00.000 --> 00:00:00.490 00:00:00.490 --> 00:00:04.790 Well, before we even knew what DNA was, much less how it was 00:00:04.790 --> 00:00:07.160 structured or it was replicated or even before we 00:00:07.160 --> 00:00:11.020 could look in and see meiosis happening in cells, we had the 00:00:11.020 --> 00:00:14.700 general sense that offspring were the products of some 00:00:14.700 --> 00:00:16.480 traits that their parents had. 00:00:16.480 --> 00:00:22.170 That if I had a guy with blue eyes-- let me say this is the 00:00:22.170 --> 00:00:27.430 blue-eyed guy right here --and then if he were to marry a 00:00:27.430 --> 00:00:32.800 brown-eyed girl-- Let's say this is the brown-eyed girl. 00:00:32.800 --> 00:00:36.410 Maybe make it a little bit more like a girl. 00:00:36.410 --> 00:00:39.450 If he were to marry the brown-eyed girl there, that 00:00:39.450 --> 00:00:42.230 most of the time, or maybe in all cases where we're dealing 00:00:42.230 --> 00:00:45.030 with the brown-eyed girl, maybe their kids are 00:00:45.030 --> 00:00:46.870 brown-eyed. 00:00:46.870 --> 00:00:50.210 Let me do this so they have a little brown-eyed baby here. 00:00:50.210 --> 00:00:53.000 00:00:53.000 --> 00:00:54.980 And this is just something-- I mean, there's obviously 00:00:54.980 --> 00:00:57.420 thousands of generations of human beings, and we've 00:00:57.420 --> 00:00:58.180 observed this. 00:00:58.180 --> 00:01:00.590 We've observed that kids look like their parents, that they 00:01:00.590 --> 00:01:05.280 inherit some traits, and that some traits seem to dominate 00:01:05.280 --> 00:01:06.140 other traits. 00:01:06.140 --> 00:01:09.960 One example of that tends to be a darker pigmentation in 00:01:09.960 --> 00:01:11.570 maybe the hair or the eyes. 00:01:11.570 --> 00:01:15.710 Even if the other parent has light pigmentation, the darker 00:01:15.710 --> 00:01:17.900 one seems to dominate, or sometimes, it actually ends up 00:01:17.900 --> 00:01:20.420 being a mix, and we've seen that all around us. 00:01:20.420 --> 00:01:23.410 Now, this study of what gets passed on and how it gets 00:01:23.410 --> 00:01:26.590 passed on, it's much older than the study of DNA, which 00:01:26.590 --> 00:01:29.840 was really kind of discovered or became a big deal in the 00:01:29.840 --> 00:01:31.110 middle of the 20th century. 00:01:31.110 --> 00:01:32.820 This was studied a long time. 00:01:32.820 --> 00:01:36.750 And kind of the father of classical genetics and 00:01:36.750 --> 00:01:38.455 heredity is Gregor Mendel. 00:01:38.455 --> 00:01:41.720 00:01:41.720 --> 00:01:45.650 He was actually a monk, and he would mess around with plants 00:01:45.650 --> 00:01:48.795 and cross them and see which traits got passed and which 00:01:48.795 --> 00:01:51.230 traits didn't get passed and tried to get an understanding 00:01:51.230 --> 00:01:55.650 of how traits are passed from one generation to another. 00:01:55.650 --> 00:02:02.090 So when we do this, when we study this classical genetics, 00:02:02.090 --> 00:02:05.230 I'm going to make a bunch of simplifying assumptions 00:02:05.230 --> 00:02:08.080 because we know that most of these don't hold for most of 00:02:08.080 --> 00:02:11.140 our genes, but it'll give us a little bit of sense of how to 00:02:11.140 --> 00:02:16.340 predict what might happen in future generations. 00:02:16.340 --> 00:02:21.270 So the first simplifying assumption I'll make is that 00:02:21.270 --> 00:02:24.840 some traits have kind of this all or nothing property. 00:02:24.840 --> 00:02:26.690 And we know that a lot of traits don't. 00:02:26.690 --> 00:02:28.690 Let's say that there are in the world-- and this is a 00:02:28.690 --> 00:02:35.220 gross oversimplification --let's say for eye color, 00:02:35.220 --> 00:02:38.520 let's say that there are two alleles. 00:02:38.520 --> 00:02:40.390 Now remember what an allele was. 00:02:40.390 --> 00:02:44.030 An allele is a specific version of a gene. 00:02:44.030 --> 00:02:48.400 So let's say that you could have blue eye color or you 00:02:48.400 --> 00:02:52.460 could have brown eye color. 00:02:52.460 --> 00:02:55.220 That we live in a universe where someone could only have 00:02:55.220 --> 00:02:58.320 one of these two versions of the eye color gene. 00:02:58.320 --> 00:03:01.230 We know that eye color is far more complex than that, so 00:03:01.230 --> 00:03:02.970 this is just a simplification. 00:03:02.970 --> 00:03:04.340 And let me just make up another one. 00:03:04.340 --> 00:03:14.110 Let me say that, I don't know, maybe for tooth size, that's a 00:03:14.110 --> 00:03:17.590 trait you won't see in any traditional biology textbook, 00:03:17.590 --> 00:03:23.470 and let's say that there's one trait for big teeth and 00:03:23.470 --> 00:03:28.330 there's another allele for small teeth. 00:03:28.330 --> 00:03:30.850 And I want to make very clear this distinction between a 00:03:30.850 --> 00:03:32.100 gene and an allele. 00:03:32.100 --> 00:03:35.230 00:03:35.230 --> 00:03:37.740 I talked about Gregor Mendel, and he was doing this in the 00:03:37.740 --> 00:03:41.890 1850s well before we knew what DNA was or what even 00:03:41.890 --> 00:03:48.590 chromosomes were and how DNA was passed on, et cetera, but 00:03:48.590 --> 00:03:53.060 let's go into the microbiology of it to understand the 00:03:53.060 --> 00:03:53.840 difference. 00:03:53.840 --> 00:03:56.170 So I have a chromosome. 00:03:56.170 --> 00:03:59.710 Let's say on some chromosome-- let me pick 00:03:59.710 --> 00:04:00.900 some chromosome here. 00:04:00.900 --> 00:04:02.930 Let's say this is some chromosome. 00:04:02.930 --> 00:04:04.590 Let's say I got that from my dad. 00:04:04.590 --> 00:04:09.130 And on this chromosome, there's some location here-- 00:04:09.130 --> 00:04:11.550 we could call that the locus on this chromosome where the 00:04:11.550 --> 00:04:15.330 eye color gene is --that's the location of 00:04:15.330 --> 00:04:16.579 the eye color gene. 00:04:16.579 --> 00:04:19.279 Now, I have two chromosomes, one from my father and one 00:04:19.279 --> 00:04:22.010 from my mother, so let's say that this is the chromosome 00:04:22.010 --> 00:04:23.260 from my mother. 00:04:23.260 --> 00:04:26.020 00:04:26.020 --> 00:04:27.800 We know that when they're normally in the cell, they 00:04:27.800 --> 00:04:30.180 aren't nice and neatly organized like this in the 00:04:30.180 --> 00:04:32.860 chromosome, but this is just to kind of show you the idea. 00:04:32.860 --> 00:04:35.690 Let's say these are homologous chromosomes so they code for 00:04:35.690 --> 00:04:36.970 the same genes. 00:04:36.970 --> 00:04:41.460 So on this gene from my mother on that same location or 00:04:41.460 --> 00:04:45.710 locus, there's also the eye color gene. 00:04:45.710 --> 00:04:51.020 Now, I might have the same version of the gene and I'm 00:04:51.020 --> 00:04:52.760 saying that there's only two versions of 00:04:52.760 --> 00:04:54.220 this gene in the world. 00:04:54.220 --> 00:04:56.690 Now, if I have the same version of the gene-- I'm 00:04:56.690 --> 00:04:58.510 going to make a little shorthand notation. 00:04:58.510 --> 00:05:01.101 I'm going to write big B-- Actually, let me do 00:05:01.101 --> 00:05:02.010 it the other way. 00:05:02.010 --> 00:05:04.040 I'm going to write little b for blue and I'm going to 00:05:04.040 --> 00:05:07.420 write big B for brown. 00:05:07.420 --> 00:05:11.210 There's a situation where this could be a little b and this 00:05:11.210 --> 00:05:12.800 could be a big B. 00:05:12.800 --> 00:05:17.010 And then I could write that my genotype-- I have the allele, 00:05:17.010 --> 00:05:20.320 I have one big B from my mom and I have one 00:05:20.320 --> 00:05:24.440 small b from my dad. 00:05:24.440 --> 00:05:29.120 Each of these instances, or ways that this gene is 00:05:29.120 --> 00:05:30.660 expressed, is an allele. 00:05:30.660 --> 00:05:40.360 So these are two different alleles-- let me write that 00:05:40.360 --> 00:05:42.880 --or versions of the same gene. 00:05:42.880 --> 00:05:46.340 And when I have two different versions like this, one 00:05:46.340 --> 00:05:50.140 version from my mom, one version from my dad, I'm 00:05:50.140 --> 00:05:53.480 called a heterozygote, or sometimes it's called a 00:05:53.480 --> 00:05:54.730 heterozygous genotype. 00:05:54.730 --> 00:06:00.410 00:06:00.410 --> 00:06:05.000 And the genotype is the exact version of the alleles I have. 00:06:05.000 --> 00:06:08.170 Let's say I had the lowercase b. 00:06:08.170 --> 00:06:11.690 I had the blue-eyed gene from both parents. 00:06:11.690 --> 00:06:16.040 So let's say that I was lowercase b, lowercase b, then 00:06:16.040 --> 00:06:18.600 I would have two identical alleles. 00:06:18.600 --> 00:06:21.840 Both of my parents gave me the same version of the gene. 00:06:21.840 --> 00:06:29.930 And this case, this genotype is homozygous, or this is a 00:06:29.930 --> 00:06:33.900 homozygous genotype, or I'm a homozygote for this trait. 00:06:33.900 --> 00:06:36.400 00:06:36.400 --> 00:06:38.510 Now, you might say, Sal, this is fine. 00:06:38.510 --> 00:06:43.400 These are the traits that you have. I have a brown from 00:06:43.400 --> 00:06:47.550 maybe my mom and a blue from my dad. 00:06:47.550 --> 00:06:50.850 In this case, I have a blue from both my mom and dad. 00:06:50.850 --> 00:06:54.590 How do we know whether my eyes are going to be brown or blue? 00:06:54.590 --> 00:06:56.900 And the reality is it's very complex. 00:06:56.900 --> 00:06:58.410 It's a whole mixture of things. 00:06:58.410 --> 00:07:02.830 But Mendel, he studied things that showed 00:07:02.830 --> 00:07:04.080 what we'll call dominance. 00:07:04.080 --> 00:07:09.260 00:07:09.260 --> 00:07:12.930 And this is the idea that one of these traits 00:07:12.930 --> 00:07:14.090 dominates the other. 00:07:14.090 --> 00:07:17.160 So a lot of people originally thought that eye color, 00:07:17.160 --> 00:07:20.490 especially blue eyes, was always dominated 00:07:20.490 --> 00:07:21.500 by the other traits. 00:07:21.500 --> 00:07:23.140 We'll assume that here, but that's a gross 00:07:23.140 --> 00:07:24.510 oversimplification. 00:07:24.510 --> 00:07:34.200 So let's say that brown eyes are dominant 00:07:34.200 --> 00:07:36.130 and blue are recessive. 00:07:36.130 --> 00:07:39.320 00:07:39.320 --> 00:07:42.960 I wanted to do that in blue. 00:07:42.960 --> 00:07:49.560 Blue eyes are recessive. 00:07:49.560 --> 00:07:52.330 If this is the case, and this is a-- As I've said 00:07:52.330 --> 00:07:55.730 repeatedly, this is a gross oversimplification. 00:07:55.730 --> 00:08:00.670 But if that is the case, then if I were to inherit this 00:08:00.670 --> 00:08:05.540 genotype, because brown eyes are dominant-- remember, I 00:08:05.540 --> 00:08:12.400 said the big B here represents brown eye and the lowercase b 00:08:12.400 --> 00:08:16.670 is recessive --all you're going to see for the person 00:08:16.670 --> 00:08:19.450 with this genotype is brown eyes. 00:08:19.450 --> 00:08:20.880 So let me do this here. 00:08:20.880 --> 00:08:21.710 Let me write this here. 00:08:21.710 --> 00:08:27.680 So genotype, and then I'll write phenotype. 00:08:27.680 --> 00:08:31.030 Genotype is the actual versions of the gene you have 00:08:31.030 --> 00:08:33.780 and then the phenotypes are what's expressed 00:08:33.780 --> 00:08:35.030 or what do you see. 00:08:35.030 --> 00:08:39.690 00:08:39.690 --> 00:08:43.860 So if I get a brown-eyed gene from my dad-- And I want to do 00:08:43.860 --> 00:08:46.310 it in a big-- I want to do it in brown. 00:08:46.310 --> 00:08:49.670 Let me do it in brown so you don't get confused. 00:08:49.670 --> 00:08:54.490 So if I've have a brown-eyed gene from my dad and a 00:08:54.490 --> 00:09:05.130 blue-eyed gene from my mom, because the brown eye is 00:09:05.130 --> 00:09:08.580 recessive, the brown-eyed allele is recessive-- And I 00:09:08.580 --> 00:09:11.370 just said a brown-eyed gene, but what I should say is the 00:09:11.370 --> 00:09:13.870 brown-eyed version of the gene, which is the brown 00:09:13.870 --> 00:09:16.820 allele, or the blue-eyed version of the gene from my 00:09:16.820 --> 00:09:18.890 mom, which is the blue allele. 00:09:18.890 --> 00:09:22.290 Since the brown allele is dominant-- I wrote that up 00:09:22.290 --> 00:09:25.410 here --what's going to be expressed are brown eyes. 00:09:25.410 --> 00:09:30.830 00:09:30.830 --> 00:09:34.450 Now, let's say I had it the other way. 00:09:34.450 --> 00:09:39.850 Let's say I got a blue-eyed allele from my dad and I get a 00:09:39.850 --> 00:09:41.750 brown-eyed allele for my mom. 00:09:41.750 --> 00:09:42.490 Same thing. 00:09:42.490 --> 00:09:46.940 The phenotype is going to be brown eyes. 00:09:46.940 --> 00:09:49.730 Now, what if I get a brown-eyed allele from both my 00:09:49.730 --> 00:09:52.470 mom and my dad? 00:09:52.470 --> 00:09:54.930 Let me see, I keep changing the shade of brown, but 00:09:54.930 --> 00:09:55.960 they're all supposed to be the same. 00:09:55.960 --> 00:09:59.130 So let's say I get two dominant brown-eyed alleles 00:09:59.130 --> 00:10:00.820 from my mom and my dad. 00:10:00.820 --> 00:10:01.770 Then what are you going to see? 00:10:01.770 --> 00:10:02.640 Well, you could guess that. 00:10:02.640 --> 00:10:08.280 I'm still going to see brown eyes. 00:10:08.280 --> 00:10:10.570 So there's only one last combination because these are 00:10:10.570 --> 00:10:12.800 the only two types of alleles we might see in our 00:10:12.800 --> 00:10:15.510 population, although for most genes, there's 00:10:15.510 --> 00:10:16.710 more than two types. 00:10:16.710 --> 00:10:18.400 For example, there's blood types. 00:10:18.400 --> 00:10:21.490 There's four types of blood. 00:10:21.490 --> 00:10:25.540 But let's say that I get two blue, one blue allele from 00:10:25.540 --> 00:10:30.400 each of my parents, one from my dad, one from my mom. 00:10:30.400 --> 00:10:33.080 Then all of a sudden, this is a recessive trait, but there's 00:10:33.080 --> 00:10:34.550 nothing to dominate it. 00:10:34.550 --> 00:10:39.130 So, all of a sudden, the phenotype will be blue eyes. 00:10:39.130 --> 00:10:42.380 And I want to repeat again, this isn't necessarily how the 00:10:42.380 --> 00:10:45.130 alleles for eye color work, but it's a nice simplification 00:10:45.130 --> 00:10:48.370 to maybe understand how heredity works. 00:10:48.370 --> 00:10:52.040 There are some traits that can be studied in this simple way. 00:10:52.040 --> 00:10:54.920 But what I wanted to do here is to show you that many 00:10:54.920 --> 00:10:58.970 different genotypes-- so these are all different genotypes 00:10:58.970 --> 00:11:02.090 --they all coded for the same phenotype. 00:11:02.090 --> 00:11:05.000 So just by looking at someone's eye color, you 00:11:05.000 --> 00:11:09.000 didn't know exactly whether they were homozygous 00:11:09.000 --> 00:11:16.740 dominant-- this would be homozygous dominant --or 00:11:16.740 --> 00:11:19.080 whether they were heterozygotes. 00:11:19.080 --> 00:11:21.350 This is heterozygous right here. 00:11:21.350 --> 00:11:23.650 These two right here are heterozygotes. 00:11:23.650 --> 00:11:27.990 00:11:27.990 --> 00:11:31.680 These are also sometimes called hybrids, but the word 00:11:31.680 --> 00:11:33.600 hybrid is kind of overloaded. 00:11:33.600 --> 00:11:36.800 It's used a lot, but in this context, it means that you got 00:11:36.800 --> 00:11:40.940 different versions of the allele for that gene. 00:11:40.940 --> 00:11:43.740 So let's think a little bit about what's actually 00:11:43.740 --> 00:11:48.155 happening when my mom and my dad reproduced. 00:11:48.155 --> 00:11:50.970 00:11:50.970 --> 00:11:53.100 Well, let's think of a couple of different scenarios. 00:11:53.100 --> 00:11:55.950 00:11:55.950 --> 00:11:57.630 Let's say that they're both hybrids. 00:11:57.630 --> 00:12:03.470 My dad has the brown-eyed dominant allele and he also 00:12:03.470 --> 00:12:08.020 has the blue-eyed recessive allele. 00:12:08.020 --> 00:12:11.560 Let's say my mom has the same thing, so brown-eyed dominant, 00:12:11.560 --> 00:12:14.780 and she also has the blue-eyed recessive allele. 00:12:14.780 --> 00:12:17.880 Now let's think about if these two people, before you see 00:12:17.880 --> 00:12:20.630 what my eye color is, if you said, look, I'm giving you 00:12:20.630 --> 00:12:22.760 what these two people's genotypes are. 00:12:22.760 --> 00:12:24.010 Let me label them. 00:12:24.010 --> 00:12:26.200 00:12:26.200 --> 00:12:27.790 Let me make this the mom. 00:12:27.790 --> 00:12:30.090 I think this is the standard convention. 00:12:30.090 --> 00:12:34.730 And let's make this right here, this is the dad. 00:12:34.730 --> 00:12:37.910 What are the different genotypes that their children 00:12:37.910 --> 00:12:38.490 could have? 00:12:38.490 --> 00:12:40.630 So let's say they reproduce. 00:12:40.630 --> 00:12:44.090 I'm going to draw a little grid here. 00:12:44.090 --> 00:12:45.660 So let me draw a grid. 00:12:45.660 --> 00:12:50.390 00:12:50.390 --> 00:12:55.990 So we know from our study of meiosis that, look, my mom has 00:12:55.990 --> 00:12:59.870 this gene on-- Let me draw the genes again. 00:12:59.870 --> 00:13:02.240 So there's a homologous pair, right? 00:13:02.240 --> 00:13:04.880 This is one chromosome right here. 00:13:04.880 --> 00:13:07.070 That's another chromosome right there. 00:13:07.070 --> 00:13:10.130 On this chromosome in the homologous pair, there might 00:13:10.130 --> 00:13:16.760 be-- at the eye color locus --there's the brown-eyed gene. 00:13:16.760 --> 00:13:19.470 And at this one, at the eye color locus, there's a 00:13:19.470 --> 00:13:20.890 blue-eyed gene. 00:13:20.890 --> 00:13:24.630 And similarly from my dad, when you look at that same 00:13:24.630 --> 00:13:28.310 chromosome in his cells-- Let me do them like this. 00:13:28.310 --> 00:13:30.740 So this is one chromosome there and this is the other 00:13:30.740 --> 00:13:32.760 chromosome here. 00:13:32.760 --> 00:13:35.120 When you look at that locus on this chromosome or that 00:13:35.120 --> 00:13:37.870 location, it has the brown-eyed allele for that 00:13:37.870 --> 00:13:40.370 gene, and on this one, it has the blue-eyed 00:13:40.370 --> 00:13:41.590 allele on this gene. 00:13:41.590 --> 00:13:44.680 And we learn from meiosis when the chromosomes-- Well, they 00:13:44.680 --> 00:13:47.580 replicate first, and so you have these two chromatids on a 00:13:47.580 --> 00:13:48.140 chromosome. 00:13:48.140 --> 00:13:51.520 But they line up in meiosis I during the metaphase. 00:13:51.520 --> 00:13:53.220 And we don't know which way they line up. 00:13:53.220 --> 00:13:56.510 For example, my dad might give me this chromosome or might 00:13:56.510 --> 00:13:57.630 give me that chromosome. 00:13:57.630 --> 00:13:59.790 Or my mom might give me that chromosome or might give me 00:13:59.790 --> 00:14:00.820 that chromosome. 00:14:00.820 --> 00:14:02.760 So I could have any of these combinations. 00:14:02.760 --> 00:14:06.540 So, for example, if I get this chromosome from my mom and 00:14:06.540 --> 00:14:09.760 this chromosome from my dad, what is the genotype going to 00:14:09.760 --> 00:14:11.000 be for eye color? 00:14:11.000 --> 00:14:16.770 Well, it's going to be capital B and capital B. 00:14:16.770 --> 00:14:21.510 If I get this chromosome from my mom and this chromosome 00:14:21.510 --> 00:14:22.620 from my dad, what's it going to be? 00:14:22.620 --> 00:14:28.330 Well, I'm going to get the big B from my dad and then I'm 00:14:28.330 --> 00:14:30.790 going to get the lowercase b from my mom. 00:14:30.790 --> 00:14:32.790 So this is another possibility. 00:14:32.790 --> 00:14:35.510 Now, this is another possibility here where I get 00:14:35.510 --> 00:14:42.490 the brown-eyed allele from my mom and I get the blue eye 00:14:42.490 --> 00:14:44.380 allele from my dad. 00:14:44.380 --> 00:14:47.350 And then there's a possibility that I get this chromosome 00:14:47.350 --> 00:14:51.260 from my dad and this chromosome from my mom, so 00:14:51.260 --> 00:14:53.520 it's this situation. 00:14:53.520 --> 00:14:55.700 Now, what are the phenotypes going to be? 00:14:55.700 --> 00:14:58.290 Well, we've already seen that this one right here is going 00:14:58.290 --> 00:15:03.080 to be brown, that one's going to be brown, this one's going 00:15:03.080 --> 00:15:06.250 to be brown, but this one is going to be blue. 00:15:06.250 --> 00:15:07.860 I already showed you this. 00:15:07.860 --> 00:15:09.980 But if I were to tell you ahead of time that, look, I 00:15:09.980 --> 00:15:11.090 have two people. 00:15:11.090 --> 00:15:13.980 They're both hybrids, or they're both heterozygotes for 00:15:13.980 --> 00:15:16.610 eye color, and eye color has this 00:15:16.610 --> 00:15:18.335 recessive dominant situation. 00:15:18.335 --> 00:15:22.530 And they're both heterozygotes where they each have one brown 00:15:22.530 --> 00:15:24.980 allele and one blue allele, and they're going to have a 00:15:24.980 --> 00:15:28.835 child, what's the probability that the child has brown eyes? 00:15:28.835 --> 00:15:35.670 00:15:35.670 --> 00:15:37.170 What's the probability? 00:15:37.170 --> 00:15:40.720 Well, each of these scenarios are equally likely, right? 00:15:40.720 --> 00:15:42.400 There's four equal scenarios. 00:15:42.400 --> 00:15:44.130 So let's put that in the denominator. 00:15:44.130 --> 00:15:45.950 Four equal scenarios. 00:15:45.950 --> 00:15:48.110 And how many of those scenarios end 00:15:48.110 --> 00:15:49.780 up with brown eyes? 00:15:49.780 --> 00:15:52.110 Well, it's one, two, three. 00:15:52.110 --> 00:15:58.780 So the probability is 3/4, or it's a 75% probability. 00:15:58.780 --> 00:16:01.830 Same logic, what's the probability that these parents 00:16:01.830 --> 00:16:04.650 produce an offspring with blue eyes? 00:16:04.650 --> 00:16:07.280 Well, that's only one of the four equally likely 00:16:07.280 --> 00:16:15.840 possibilities, so blue eyes is only 25%. 00:16:15.840 --> 00:16:19.400 Now, what is the probability that they produce a 00:16:19.400 --> 00:16:20.390 heterozygote? 00:16:20.390 --> 00:16:23.130 So what is the probability that they produce a 00:16:23.130 --> 00:16:24.425 heterozygous offspring? 00:16:24.425 --> 00:16:27.360 00:16:27.360 --> 00:16:29.300 So now we're not looking at the phenotype anymore. 00:16:29.300 --> 00:16:31.050 We're looking at the genotype. 00:16:31.050 --> 00:16:34.310 So of these combinations, which are heterozygous? 00:16:34.310 --> 00:16:36.570 Well, this one is, because it has a mix. 00:16:36.570 --> 00:16:37.360 It's a hybrid. 00:16:37.360 --> 00:16:39.380 It has a mix of the two alleles. 00:16:39.380 --> 00:16:41.170 And so is this one. 00:16:41.170 --> 00:16:42.220 So what's the probability? 00:16:42.220 --> 00:16:45.050 Well, there's four different combinations. 00:16:45.050 --> 00:16:48.020 All of those are equally likely, and two of them result 00:16:48.020 --> 00:16:49.200 in a heterozygote. 00:16:49.200 --> 00:16:54.580 So it's 2/4 or 1/2 or 50%. 00:16:54.580 --> 00:16:56.570 So using this Punnett square, and, of course, we had to make 00:16:56.570 --> 00:16:59.880 a lot of assumptions about the genes and whether one's 00:16:59.880 --> 00:17:02.050 dominant or one's a recessive, we can start to make 00:17:02.050 --> 00:17:03.880 predictions about the probabilities 00:17:03.880 --> 00:17:05.530 of different outcomes. 00:17:05.530 --> 00:17:07.300 And as we'll see in future videos, you can actually even 00:17:07.300 --> 00:17:07.970 go backwards. 00:17:07.970 --> 00:17:10.680 You can say, hey, given that this couple had five kids with 00:17:10.680 --> 00:17:14.160 brown eyes, what's the probability that they're both 00:17:14.160 --> 00:17:15.819 heterozygotes, or something like that. 00:17:15.819 --> 00:17:19.000 So it's a really interesting area, even though it is a bit 00:17:19.000 --> 00:17:20.490 of oversimplification. 00:17:20.490 --> 00:17:23.760 But many traits, especially some of the things that Gregor 00:17:23.760 --> 00:17:27.190 Mendel studied, can be studied in this way.