WEBVTT 00:00:14.756 --> 00:00:16.217 These days scientists know 00:00:16.241 --> 00:00:18.661 how you inherit characteristics from your parents. 00:00:18.685 --> 00:00:21.791 They're able to calculate probabilities of having a specific trait 00:00:21.815 --> 00:00:23.156 or getting a genetic disease 00:00:23.180 --> 00:00:26.430 according to the information from the parents and the family history. 00:00:26.454 --> 00:00:27.913 But how is this possible? 00:00:27.937 --> 00:00:31.321 To understand how traits pass from one living being to its descendants, 00:00:31.345 --> 00:00:33.536 we need to go back in time to the 19th century 00:00:33.560 --> 00:00:35.286 and a man named Gregor Mendel. 00:00:35.849 --> 00:00:37.869 Mendel was an Austrian monk and biologist 00:00:37.893 --> 00:00:39.591 who loved to work with plants. 00:00:39.615 --> 00:00:42.837 By breeding the pea plants he was growing in the monastery's garden, 00:00:42.861 --> 00:00:45.108 he discovered the principles that rule heredity. 00:00:45.132 --> 00:00:46.668 In one of most classic examples, 00:00:46.692 --> 00:00:48.875 Mendel combined a purebred yellow-seeded plant 00:00:48.899 --> 00:00:50.944 with a purebred green-seeded plant, 00:00:50.968 --> 00:00:52.897 and he got only yellow seeds. 00:00:52.921 --> 00:00:55.357 He called the yellow-colored trait the dominant one, 00:00:55.381 --> 00:00:57.554 because it was expressed in all the new seeds. 00:00:58.011 --> 00:01:01.356 Then he let the new yellow-seeded hybrid plants self-fertilize. 00:01:01.380 --> 00:01:04.500 And in this second generation, he got both yellow and green seeds, 00:01:04.524 --> 00:01:07.683 which meant the green trait had been hidden by the dominant yellow. 00:01:07.707 --> 00:01:09.989 He called this hidden trait the recessive trait. 00:01:10.013 --> 00:01:11.691 From those results, Mendel inferred 00:01:11.715 --> 00:01:13.903 that each trait depends on a pair of factors, 00:01:13.927 --> 00:01:15.562 one of them coming from the mother 00:01:15.586 --> 00:01:17.081 and the other from the father. 00:01:17.105 --> 00:01:19.416 Now we know that these factors are called alleles 00:01:19.440 --> 00:01:21.780 and represent the different variations of a gene. 00:01:21.804 --> 00:01:24.633 Depending on which type of allele Mendel found in each seed, 00:01:24.657 --> 00:01:28.231 we can have what we call a homozygous pea, where both alleles are identical, 00:01:28.255 --> 00:01:29.986 and what we call a heterozygous pea, 00:01:30.010 --> 00:01:31.732 when the two alleles are different. 00:01:31.756 --> 00:01:34.470 This combination of alleles is known as genotype 00:01:34.494 --> 00:01:36.446 and its result, being yellow or green, 00:01:36.470 --> 00:01:38.184 is called phenotype. 00:01:38.208 --> 00:01:41.483 To clearly visualize how alleles are distributed amongst descendants, 00:01:41.507 --> 00:01:43.526 we can a diagram called the Punnett square. 00:01:43.550 --> 00:01:45.631 You place the different alleles on both axes 00:01:45.655 --> 00:01:47.806 and then figure out the possible combinations. 00:01:47.830 --> 00:01:49.813 Let's look at Mendel's peas, for example. 00:01:49.837 --> 00:01:52.883 Let's write the dominant yellow allele as an uppercase "Y" 00:01:52.907 --> 00:01:55.471 and the recessive green allele as a lowercase "y." 00:01:55.495 --> 00:01:58.143 The uppercase Y always overpowers his lowercase friend, 00:01:58.167 --> 00:01:59.955 so the only time you get green babies 00:01:59.979 --> 00:02:01.731 is if you have lowercase Y's. 00:02:02.207 --> 00:02:05.004 In Mendel's first generation, the yellow homozygous pea mom 00:02:05.028 --> 00:02:07.422 will give each pea kid a yellow-dominant allele, 00:02:07.446 --> 00:02:10.667 and the green homozygous pea dad will give a green-recessive allele. 00:02:10.691 --> 00:02:13.090 So all the pea kids will be yellow heterozygous. 00:02:13.637 --> 00:02:15.232 Then, in the second generation, 00:02:15.256 --> 00:02:17.067 where the two heterozygous kids marry, 00:02:17.091 --> 00:02:20.001 their babies could have any of the three possible genotypes, 00:02:20.025 --> 00:02:21.728 showing the two possible phenotypes 00:02:21.752 --> 00:02:23.401 in a three-to-one proportion. 00:02:23.838 --> 00:02:25.891 But even peas have a lot of characteristics. 00:02:25.915 --> 00:02:27.931 For example, besides being yellow or green, 00:02:27.955 --> 00:02:29.517 peas may be round or wrinkled. 00:02:29.541 --> 00:02:31.835 So we could have all these possible combinations: 00:02:31.859 --> 00:02:33.590 round yellow peas, round green peas, 00:02:33.614 --> 00:02:35.632 wrinkled yellow peas, wrinkled green peas. 00:02:35.656 --> 00:02:38.538 To calculate the proportions for each genotype and phenotype, 00:02:38.562 --> 00:02:40.105 we can use a Punnett square too. 00:02:40.129 --> 00:02:42.514 Of course, this will make it a little more complex. 00:02:42.538 --> 00:02:44.885 And lots of things are more complicated than peas, 00:02:44.909 --> 00:02:46.304 like, say, people. 00:02:46.790 --> 00:02:49.980 These days, scientists know a lot more about genetics and heredity. 00:02:50.004 --> 00:02:53.534 And there are many other ways in which some characteristics are inherited. 00:02:53.558 --> 00:02:55.730 But, it all started with Mendel and his peas.