1 00:00:00,000 --> 00:00:06,104 The principles of inheritance in classical genetics is the topic of this lecture. 2 00:00:06,104 --> 00:00:15,905 The lecture is part of Module 1, Animal Genetics. The creation of this presentation was supported by the ERASMUS+ KA2 grant 3 00:00:15,905 --> 00:00:26,861 as part of the ISAGREED project, Innovation of Content and Structure of Study Programs in the Field of Livestock Genetic and Food Resources Management Using Digitalization. 4 00:00:28,115 --> 00:00:36,002 We will be discussing Mendel's experiment. What was his aim? What methods did he use? 5 00:00:36,002 --> 00:00:42,305 How did he evaluate it? And, most importantly, what can be concluded from his experiments? 6 00:00:42,305 --> 00:00:50,654 We can find the motivation for Mendel's experiment in the introduction of his article from 1866. 7 00:00:50,654 --> 00:01:01,907 His motivation was to find out what causes the diversity in colors of ornamental plants when utilizing hybridization (artificial pollination). 8 00:01:01,907 --> 00:01:11,345 Notice that he does not mention anything about inheritance. However, it can be implied that he was indeed interested in inheritance. 9 00:01:11,345 --> 00:01:23,357 The methodology is important, particularly these points. He chose an experimental organism that had to fulfill certain conditions, 10 00:01:23,357 --> 00:01:28,208 such as high fertility, self-pollination, and having simple traits. 11 00:01:28,208 --> 00:01:36,524 These traits are alternative traits, meaning traits that have either one variant or the other. 12 00:01:36,524 --> 00:01:50,483 He ultimately chose pea plants. He conducted two years of creating pure lines with these plants, obtaining two parent lines, each with one variant of the trait. 13 00:01:50,483 --> 00:02:03,617 He then intentionally crossed the parent lines. He transferred pollen grains from one set of parents to the stigma of the other set of parents using a brush. 14 00:02:03,617 --> 00:02:09,425 He observed what he obtained in the subsequent generations of offspring. 15 00:02:09,425 --> 00:02:18,863 He always recorded the numbers of individuals with specific traits in each generation. He mathematically evaluated 16 00:02:18,863 --> 00:02:26,585 the data using probability theory. From the obtained data, he inferred how inheritance occurs. 17 00:02:27,575 --> 00:02:37,706 Mendel studied at the University of Vienna, where he encountered new scientific knowledge and methods. Among other things, 18 00:02:37,706 --> 00:02:46,319 he realized the fundamental reproductive principles of higher organisms. He based his work on the basic principle of reproduction, 19 00:02:46,319 --> 00:03:01,367 which requires the fusion of male gametes, or pollen grains, with female gametes, or eggs. Both types of gametes are equally important. 20 00:03:01,367 --> 00:03:12,686 He assumed that in order for parents and offspring to resemble each other, parents must pass something to their offspring through the gametes. 21 00:03:12,686 --> 00:03:24,929 What is present in the gametes must have half the amount of hereditary material. The fusion of gametes results in a zygote, 22 00:03:24,929 --> 00:03:36,149 an offspring, which must inevitably have the same amount of this hereditary material as the parents. Nowadays, 23 00:03:36,149 --> 00:03:42,881 we don't find this surprising because we understand that higher organisms are usually diploid. 24 00:03:42,881 --> 00:03:53,177 In order for diploid parents to produce diploid offspring, there must be a reduction of the hereditary material by half in the gametes. 25 00:03:53,177 --> 00:04:04,760 From this assumption, Mendel likely derived the basic idea, hypothesis, that what is inherited must be a discrete element, a unit. 26 00:04:04,760 --> 00:04:11,723 Each individual has two of these units, and there must be only one unit in the gametes. 27 00:04:11,723 --> 00:04:25,220 Towards the end of his work, he called these hereditary units "die Elementen". Mendel chose 7 pairs of contrasting traits in peas, 28 00:04:25,220 --> 00:04:36,407 each with only two variants. One example is seed shape - round or wrinkled. Another example is seed color - yellow or green. 29 00:04:36,407 --> 00:04:43,964 Mendel noticed during the crosses that one variant dominated over the other in the case of hybrids. 30 00:04:43,964 --> 00:04:55,019 The terms dominance and recessiveness come from Mendel. We will describe 2 types of crossings. 31 00:04:55,019 --> 00:05:05,777 The first one is monohybrid crossing. In the parental generation, phenotypically different parents in a particular trait were deliberately crossed. 32 00:05:05,777 --> 00:05:15,842 We will choose only the pea color. So we have parents with only yellow or only green peas. Mendel created female plants 33 00:05:15,842 --> 00:05:24,389 from one variant (e.g. yellow color) (by removing immature stamens) and only had female flowers. 34 00:05:24,389 --> 00:05:34,486 From the other parents (green color), he then transferred pollen grains to the stigma of the castrated plants using a brush. 35 00:05:34,486 --> 00:05:44,584 This way, he had control over what is being transferred where. After fertilization, he found in the F1 generation 36 00:05:44,584 --> 00:05:59,038 that all offspring had only one type of trait, which was the dominant one, with only yellow peas - we speak of phenotypic uniformity. 37 00:05:59,038 --> 00:06:11,842 When he allowed this F1 generation to reproduce freely among themselves, he found that both plants with yellow and green seed color appeared 38 00:06:11,842 --> 00:06:21,874 in the population of offspring in the F2 generation. There was no dilution of colors, only those two basic colors were present. 39 00:06:21,874 --> 00:06:30,751 But when he counted them, he found that the ratio of yellow to green peas was 3:1. 40 00:06:30,751 --> 00:06:38,407 And he obtained this ratio in the same experiment for all seven traits. This is not a coincidence. 41 00:06:38,407 --> 00:06:51,244 How did Mendel explain this? He used his hypothesis of the discreteness of the hereditary material and identified elements 42 00:06:51,244 --> 00:06:59,263 for the dominant trait with a capital letter "A" and for the recessive trait with a lowercase letter "a". If parents pass 43 00:06:59,263 --> 00:07:10,153 on these elements through gametes to their offspring, then both parents and offspring must have these elements in pairs and only one in gametes. 44 00:07:10,153 --> 00:07:20,878 And because he selected pure lines for parents, they must carry both the same elements - today we call them dominant and recessive homozygotes. 45 00:07:20,878 --> 00:07:27,775 Dominant homozygous parents with yellow peas only produce gametes with the dominant element - today called an allele. 46 00:07:27,775 --> 00:07:42,163 gametes with the recessive allele. When the gametes combine, hybrid individuals with both the dominant allele and the recessive allele "Aa" 47 00:07:42,163 --> 00:07:50,545 necessarily arise - today we call them heterozygous genotypes. Mendel called them hybrids 48 00:07:50,545 --> 00:08:01,930 This genotypic uniformity causes the emergence of phenotypic uniformity. Only one variant, the dominant one, is expressed. 49 00:08:01,930 --> 00:08:10,279 The recessive trait, green color, does not appear. The expression of the recessive allele is suppressed. 50 00:08:10,279 --> 00:08:22,489 But it is present in individuals. When F1 individuals, heterozygotes, reproduce among themselves, gametes are formed again. 51 00:08:22,489 --> 00:08:32,455 But this time it is necessary to take into account that those two alleles are different and that gametes will be formed with both 52 00:08:32,455 --> 00:08:42,982 the dominant allele A and the recessive allele. Mendel called this segregation. In each gamete of hybrid individuals, 53 00:08:42,982 --> 00:08:53,938 there is either the first or the second variant. These gametes are formed in large quantities and therefore in a ratio of 1:1. 54 00:08:53,938 --> 00:09:07,666 And this is the key to explaining the ratios he found in the F2 generation. Male and female gametes are now formed in a 50%: 50% ratio 55 00:09:07,666 --> 00:09:17,104 with the dominant allele to the recessive allele. The fusion of gametes during fertilization is determined by the probability 56 00:09:17,104 --> 00:09:25,882 of the fusion of these gametes multiplied by the probability of the occurrence of these gametes in the genofond of the parents. 57 00:09:25,882 --> 00:09:38,686 This creates again dominant homozygotes in the F2 generation in a ratio of ¼ (because ½ of female gametes have allele A 58 00:09:38,686 --> 00:09:54,196 and ½ of male gametes have the recessive allele, which is ½ * ½ = ¼). And this applies to every combination of parental gametes. 59 00:09:54,196 --> 00:10:05,482 The results are best shown by Punnett's combination square. And that is why dominant allele can also be combined with the recessive one, 60 00:10:05,482 --> 00:10:19,177 resulting in two variants of heterozygotes Aa. BUT two recessive alleles can also combine, again with a probability of ¼. 61 00:10:19,177 --> 00:10:32,839 And that is the reason why the recessive trait, recessive homozygotes, reappear in the F2 generation. And when we add up these probabilities, 62 00:10:32,839 --> 00:10:47,887 the ratio of 3:1 is obtained. So nothing is random. That is important. Inheritance has its rules and these rules come from the mathematical 63 00:10:47,887 --> 00:10:57,556 probability of combining parental gametes to create a specific genotype of offspring. 64 00:10:58,315 --> 00:11:08,874 Although Mendel never mentioned the term "inheritance" in his work, from his experiments and how he interpreted them, it follows 65 00:11:08,874 --> 00:11:20,754 that Mendel was the first to understand how inheritance works, what and how is inherited. That inheritance is about the transfer of these elements 66 00:11:20,754 --> 00:11:32,535 through gametes to offspring. In other words, genetic information is not diluted, mixed, on the contrary, it is discrete and constant. 67 00:11:33,228 --> 00:11:44,019 And when we project it back into the initial scheme of reproduction and add sets of chromosomes, whose existence and 68 00:11:44,019 --> 00:11:53,523 function in inheritance Mendel did not know at that time, we see that it fits together, both the elements, alleles, 69 00:11:53,523 --> 00:11:57,450 and the behavior of chromosomes during gamete formation. 70 00:11:59,430 --> 00:12:07,647 These are the basic three Mendelian principles that can be derived from monohybrid crosses: 71 00:12:07,647 --> 00:12:17,910 Genetic characteristics are controlled by unit factors, which exist in pairs (genotypes) in individual organisms. 72 00:12:17,910 --> 00:12:27,909 The second principle is dominance and recessiveness. It refers to the expression of the phenotype in heterozygous individuals, 73 00:12:27,909 --> 00:12:38,766 where only one of the traits is manifested in the phenotype - the dominant one. The recessive allele remains in those individuals, 74 00:12:38,766 --> 00:12:45,762 but it is not expressed, although it can be passed on to the next generation. 75 00:12:45,762 --> 00:12:54,804 The main principle is the principle of segregation, which tells us that during the formation of gametes, the paired unit factors separate, 76 00:12:54,804 --> 00:13:03,912 so that each gamete receives one or the other element with equal probability. 77 00:13:07,179 --> 00:13:16,485 In the dihybrid experiment, Mendel observed the variability of two different traits simultaneously in the same experimental 78 00:13:16,485 --> 00:13:19,455 design as in monohybrid crosses. 79 00:13:21,006 --> 00:13:35,163 Let's use an example of two traits: seed color and seed shape. The dominant color is yellow, the recessive color is green; the dominant shape is round, and the recessive shape is wrinkled. 80 00:13:35,163 --> 00:13:44,271 It didn't matter if the parental plants carried both traits as dominant or if one parent carried both traits as recessive. 81 00:13:44,271 --> 00:13:54,567 Or if the parents carried different combinations of dominant and recessive variants for both traits (one dominant and the other recessive). 82 00:13:54,567 --> 00:14:05,556 They were still pure lines, i.e., homozygotes. In the F1 generation, Mendel again observed phenotypic uniformity. 83 00:14:05,556 --> 00:14:16,974 All individuals carried both dominant traits - yellow color and round seeds. And when the F1 individuals were crossed with each other, 84 00:14:16,974 --> 00:14:25,224 a specific ratio of 9:3:3:1 was obtained again in the F2 generation. 85 00:14:25,224 --> 00:14:36,840 This happened every time he chose two different traits out of the seven. How did Mendel explain this? He again used symbols 86 00:14:36,840 --> 00:14:47,367 of capital and small letters to represent dominant and recessive alleles, and determined which alleles for both traits could appear in the gametes. 87 00:14:47,367 --> 00:14:59,478 And because both types of parents were pure lines, i.e., homozygotes, each parent produced only one type of gamete. And when they combined, 88 00:14:59,478 --> 00:15:10,236 heterozygous individuals were always produced in the F1 generation. In the F1, we have dihybrids, double heterozygotes, 89 00:15:10,236 --> 00:15:13,206 and these were crossed with each other. 90 00:15:14,955 --> 00:15:25,548 The result is best described using this combination table. Heterozygous parents of the F1 generation YyRr can produce 4 types of gametes 91 00:15:25,548 --> 00:15:39,144 that combine one allele from each partial genotype, the Y gene for color and the R gene for seed shape. The alleles of the two genes combine freely, 92 00:15:39,144 --> 00:15:53,334 resulting in a ratio of 1:1:1:1. Each type of gamete is formed with a probability of ¼. When these male and female gametes are combined during fertilization, 93 00:15:53,334 --> 00:16:04,422 new combinations of genotypes of the F2 generation offspring are formed. And when we determine their phenotype based on their genotypes, 94 00:16:04,422 --> 00:16:15,378 we must obtain the ratio 9 3 3 1. These two different traits are genetically independent, and this is precisely manifested in the free 95 00:16:15,378 --> 00:16:27,093 combinability of alleles of different genes during gamete formation. This conclusion is called the principle of independent assortment. 96 00:16:27,093 --> 00:16:43,856 The summary of these of the dihybrid experiment we can see here.In F2 generation Mendel get four types of combination of phenotypes in ration 9 : 3 : 3 : 1. 97 00:16:46,133 --> 00:16:59,003 Mendel, as a true scientist, tested his hypothesis through test crosses, in which he crossed heterozygotes with recessive homozygotes from the parental generation. 98 00:16:59,003 --> 00:17:10,289 Because recessive homozygotes form only one type of gamete and heterozygotes form 4 types of gametes in a ratio of 1:1:1:1, 99 00:17:10,289 --> 00:17:19,826 all 4 types of phenotypic combinations appear in the test offspring in a ratio of 1:1:1:1. 100 00:17:22,565 --> 00:17:27,779 The rule of independent assortment is a conclusion from Mendel's dihybrid cross experiments. 101 00:17:27,779 --> 00:17:46,028 During gamete formation, different pairs of unit factors segregate independently of each other (alleles of one allelic pair are independent of the segregation of alleles of the second allelic pair). 102 00:17:48,107 --> 00:17:54,443 From Mendel's experiments and his conclusions, the most important thing is evident. 103 00:17:54,542 --> 00:18:01,604 This is why Mendel is considered the father of genetics, as he first understood how inheritance works. 104 00:18:01,604 --> 00:18:11,273 Mendel understood that traits are not inherited. Traits do not inherit. What is inherited is what is in the gametes; 105 00:18:11,273 --> 00:18:21,701 there are no traits there. What is inherited are precisely those discrete genetic elements, which we now call genes, 106 00:18:21,701 --> 00:18:29,423 and which are found in one variant in each gamete. 107 00:18:30,941 --> 00:18:33,911 And thank you for your attention.