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These are the user uploaded subtitles that are being translated: 0 00:00:00,445 --> 00:00:01,400 MICHELLE: Hi everyone. 1 00:00:01,400 --> 00:00:01,990 Welcome back. 2 00:00:01,990 --> 00:00:05,100 It's Michelle here again, and today, we're going to continue our discussion 3 00:00:05,100 --> 00:00:06,550 of meiosis. 4 00:00:06,550 --> 00:00:09,690 Today we'll be able to connect meiosis to Mendel's Law of Independent 5 00:00:09,690 --> 00:00:10,520 Assortment. 6 00:00:10,520 --> 00:00:12,910 Let's get started. 7 00:00:12,910 --> 00:00:13,150 OK. 8 00:00:13,150 --> 00:00:16,129 Here you see a diagram much like the ones that we saw in the previous 9 00:00:16,129 --> 00:00:21,170 video, except this one is a nucleus from a diploid cell with two different 10 00:00:21,170 --> 00:00:22,420 chromosomes. 11 00:00:22,420 --> 00:00:25,220 That means that 2n equals 4. 12 00:00:25,220 --> 00:00:27,660 Remember that this means that the organism contains 13 00:00:27,660 --> 00:00:29,600 four chromosomes total-- 14 00:00:29,600 --> 00:00:34,090 two copies of the long chromosome and two copies of the short chromosome. 15 00:00:34,090 --> 00:00:38,490 Let's imagine that the red chromosomes were inherited from mom, and the blue 16 00:00:38,490 --> 00:00:40,900 chromosomes were inherited from dad. 17 00:00:40,900 --> 00:00:45,190 I'd next like you to assume that this individual has the genotype big A 18 00:00:45,190 --> 00:00:47,580 little a, big B little b. 19 00:00:47,580 --> 00:00:50,810 So we're going to consider the segregation of the alleles of two 20 00:00:50,810 --> 00:00:54,670 genes, the A gene and the B gene, for this example. 21 00:00:54,670 --> 00:01:00,260 Next imagine that this heterozygote had a mom whose genotype was big A big 22 00:01:00,260 --> 00:01:05,330 A, little b little b, and then dad, who had the genotype little a little 23 00:01:05,330 --> 00:01:09,210 a, big B big B. What does that mean? 24 00:01:09,210 --> 00:01:16,460 Well, that means that mom contributed big A and little b in her gametes, and 25 00:01:16,460 --> 00:01:20,970 dad contributed little a and big B in his gametes. 26 00:01:20,970 --> 00:01:21,180 OK. 27 00:01:21,180 --> 00:01:24,350 Next I'd like you to assume that the A and B loci are on different 28 00:01:24,350 --> 00:01:25,390 chromosomes. 29 00:01:25,390 --> 00:01:29,080 So given that information, I'd like you to take a minute, and with paper 30 00:01:29,080 --> 00:01:32,560 and pencil or through a computer or however you choose, recreate this 31 00:01:32,560 --> 00:01:35,420 figure for yourself and label the chromosomes with 32 00:01:35,420 --> 00:01:37,440 all the A and B alleles. 33 00:01:37,440 --> 00:01:40,570 Pause this video, and then we'll come back and work on it together. 34 00:01:46,745 --> 00:01:47,220 Great. 35 00:01:47,220 --> 00:01:48,350 Let's see how you did. 36 00:01:48,350 --> 00:01:51,380 So although we know that the A locus and the B locus are on different 37 00:01:51,380 --> 00:01:53,860 chromosomes, we don't know where on that chromosome. 38 00:01:53,860 --> 00:01:58,240 So for the purposes of this example, we're just going to pick a spot. 39 00:01:58,240 --> 00:02:01,810 But we do know that the red chromosomes are from mom, and the blue 40 00:02:01,810 --> 00:02:03,450 chromosomes are from dad. 41 00:02:03,450 --> 00:02:09,410 Remember that mom contributed big A. So let's just place big A allele here. 42 00:02:09,410 --> 00:02:13,340 If we look at the homologous chromosome that you got from dad, at 43 00:02:13,340 --> 00:02:16,290 the same locus, the allele is little a. 44 00:02:16,290 --> 00:02:21,850 Likewise, let's imagine that this is the locus for the B gene. 45 00:02:21,850 --> 00:02:24,180 Mom contributed a little b allele. 46 00:02:24,180 --> 00:02:29,800 And at the same locus on the homolog, dad contributed the big B allele. 47 00:02:29,800 --> 00:02:30,630 Great. 48 00:02:30,630 --> 00:02:34,000 So now I'd like you to pause the video for a minute and trace these alleles 49 00:02:34,000 --> 00:02:35,740 through every stage of meiosis. 50 00:02:35,740 --> 00:02:37,790 What's the genotypes of the gametes that you get? 51 00:02:42,860 --> 00:02:43,370 Excellent. 52 00:02:43,370 --> 00:02:45,210 Let's go through this together. 53 00:02:45,210 --> 00:02:50,940 So the first step of meiosis is replication, and so here we have the 54 00:02:50,940 --> 00:02:52,770 replicated chromosomes. 55 00:02:52,770 --> 00:02:56,850 And remember that sisters are pretty much copies of each other, so if you 56 00:02:56,850 --> 00:03:00,990 placed the alleles on the replicated chromosomes, it's going to look 57 00:03:00,990 --> 00:03:02,540 something like this. 58 00:03:02,540 --> 00:03:03,530 OK. 59 00:03:03,530 --> 00:03:04,060 Great. 60 00:03:04,060 --> 00:03:06,760 So now the next step, again, is recombination. 61 00:03:06,760 --> 00:03:12,260 After recombination occurs, the chromosomes align in meiosis I. So I'm 62 00:03:12,260 --> 00:03:13,750 going to show that here. 63 00:03:13,750 --> 00:03:13,990 OK. 64 00:03:13,990 --> 00:03:18,860 So here we are in metaphase I, and I'm just going to quickly place the 65 00:03:18,860 --> 00:03:20,630 alleles on this diagram. 66 00:03:20,630 --> 00:03:25,470 I'm going to label both sisters at once for simplicity. 67 00:03:25,470 --> 00:03:30,140 Is this the only way that the chromosomes can align in metaphase I? 68 00:03:30,140 --> 00:03:34,660 Is there a specific reason why I put the red chromosomes on the left side 69 00:03:34,660 --> 00:03:36,560 and the blue chromosomes on the other? 70 00:03:36,560 --> 00:03:37,470 There's not. 71 00:03:37,470 --> 00:03:41,370 The alignment of chromosomes in metaphase I is random. 72 00:03:41,370 --> 00:03:44,470 And that means that here is one possible 73 00:03:44,470 --> 00:03:46,460 configuration, but there's another. 74 00:03:46,460 --> 00:03:51,190 Chromosomes can also align like this at metaphase I. In this alignment, the 75 00:03:51,190 --> 00:03:52,960 alleles will look like this. 76 00:03:52,960 --> 00:03:55,870 What does this mean for meiotic chromosome segregation? 77 00:03:55,870 --> 00:03:59,695 Let's first take a look at the meiosis on the left panel. 78 00:03:59,695 --> 00:04:05,150 So homologous chromosomes segregate in meiosis I, and sisters split in 79 00:04:05,150 --> 00:04:06,530 meiosis II. 80 00:04:06,530 --> 00:04:08,660 And so you get two types of gametes-- 81 00:04:08,660 --> 00:04:15,060 gametes with a little b big A genotype or a little a big B genotype, like so. 82 00:04:15,060 --> 00:04:19,070 So now what does this mean for meiosis if chromosomes align in the other way? 83 00:04:19,070 --> 00:04:20,660 And so that I'm showing here. 84 00:04:20,660 --> 00:04:23,860 Let me just take a minute to place the alleles. 85 00:04:23,860 --> 00:04:27,740 And now sisters split in meiosis II. 86 00:04:27,740 --> 00:04:33,280 So if chromosomes align like this, you'll get gametes of genotype big A 87 00:04:33,280 --> 00:04:36,210 big B and little a little b. 88 00:04:36,210 --> 00:04:41,310 So in this population of gametes, you would expect equal frequencies of 89 00:04:41,310 --> 00:04:42,530 these genotypes-- 90 00:04:42,530 --> 00:04:48,460 big A little b, little a big B, big A big B, and little a little b. 91 00:04:48,460 --> 00:04:52,430 Now let's think about a Punnett square for a dihybrid cross, where one of the 92 00:04:52,430 --> 00:04:56,660 individuals has genotype big A little a and big B little b. 93 00:04:56,660 --> 00:05:01,310 So we write that the four combinations of gametes would look like this, and 94 00:05:01,310 --> 00:05:03,980 now we can understand why this would be. 95 00:05:03,980 --> 00:05:07,680 If chromosomes align in the orientation on the left, you get big A 96 00:05:07,680 --> 00:05:13,070 little b and little a big B, which correspond to these two gametes. 97 00:05:13,070 --> 00:05:16,500 But in the other half of the population, you'll align in the other 98 00:05:16,500 --> 00:05:20,950 orientation and get out gametes that have genotype big A big B and 99 00:05:20,950 --> 00:05:22,480 little a little b. 100 00:05:22,480 --> 00:05:26,620 And these will come out at equal frequencies when the genes are located 101 00:05:26,620 --> 00:05:29,780 on different chromosomes and, therefore, assort independently. 102 00:05:29,780 --> 00:05:34,190 So to predict all the possible gametes when considering three or more genes 103 00:05:34,190 --> 00:05:35,690 becomes very complicated. 104 00:05:35,690 --> 00:05:39,340 And when you consider the thousands of genes that humans have on their 23 105 00:05:39,340 --> 00:05:43,220 pairs of chromosomes, it becomes easier to understand why siblings can 106 00:05:43,220 --> 00:05:46,660 appear very different from each other. 107 00:05:46,660 --> 00:05:47,680 Great work, everyone. 108 00:05:47,680 --> 00:05:50,020 We've gone through so many concepts already. 109 00:05:50,020 --> 00:05:53,410 And hopefully now you understand the connection between meiosis and 110 00:05:53,410 --> 00:05:55,600 Mendel's Law of Independent Assortment. 111 00:05:55,600 --> 00:05:56,850 See you next time. 9188

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