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Transcript
Mendel’s Discoveries Slide 2 Gregor Mendel was an Austrian monk who lived in the middle of the 19th century. A fair amount about Mendel’s life is known from history, including the fact that some of his lowest grades received in school were in biology. Despite his academic shortcomings, however, Mendel’s work as a scientist uncovered some of the most fundamental concepts of genetics, and provided a basis for many of the major advancements in biology that were to come in the following century. Slide 3 Over a period of about nine years, Mendel performed breeding experiments on bean plants by collecting data on the hereditary patterns of a number of different characters of the plants, such as the color of fruits and flowers, the shape of seeds and fruits, and the size of plants. Mendel’s studies involved crossing bean plants that were true-breeding for different forms of various characters. For example, one of Mendel’s crosses involved breeding plants that always produced smooth seeds with plants that were true for wrinkled seeds. The outcome of this cross is shown in the illustration. Mendel called his true-breeding plants the parental generation, or P. Crosses between two parents result in the first filial generation, or F1, for short. These F1 progeny were then allowed to self-fertilize, producing a second filial generation, or F2. Mendel measured the characters under study as they occurred in both the F1 and F2 generations, and then used a fairly simple mathematical explanation to describe the results that he found. As we will discuss in the next few slides, Mendel’s results led to the discovery of two fundamental laws that govern heredity not just in bean plants, but in all sexually reproducing organisms. Slide 4 Mendel’s first experiments described the heredity patterns of single characters, such as flower color, or seed shape. Crosses performed to follow the inheritance of a single character are called monohybrid crosses. Invariably in these crosses, the F1 generation displayed only one trait, or version of the character, that was present in the parental generation. For example, when Mendel crossed purple-flowered plants with white-flowered plants, only purple-flowered plants were observed in the F1 generation. When plants with smooth seeds were crossed with plants with wrinkled seeds, only smooth-seeded plants were observed in the F1 generation. Mendel called the traits that appeared in the F1 generation dominant traits. The traits that did not appear in the F1 generation were called recessive traits. When Mendel allowed the F1 generation to self-pollinate, the recessive traits reappeared in the resulting F2 generation. However, as shown in the table, the dominant and recessive traits always appeared in the ratio of three dominant phenotypes to one recessive phenotype. Slide 5 Mendel explained these results by reasoning that each plant had two units of inheritance for any given trait. Plants could then theoretically have three possible combinations for a character - they could have two dominant units, two recessive units, or a dominant and a recessive unit. Furthermore, when the adult plants formed gametes through the process of meiosis, each of the units separated so that each gamete carried only one unit. This means that when gametes from separate parents combined and developed into a new plant, the offspring would have one unit of inheritance for a character from each of its parents. Today, we call Mendel’s units of inheritance genes, and the different versions – dominant and recessive – different alleles of the same gene. As you follow through this lesson, note that different genes are denoted by letters. The dominant allele of a gene is represented by a capital letter, while the recessive allele is denoted by a lower-case letter. For example, the gene for flower color is represented by the letter ‘w’. The dominant allele that results in purple color is a capital W; the recessive form that corresponds to white color is a lower case w. Individuals with two of the same alleles are termed homozygous, and are represented as WW or ww, while individuals with different alleles are termed heterozygous, and are represented as Ww. Mendel’s true-breeding plants, therefore, were homozygous for certain characters, such as flower color. When these homozygous parental plants produced gametes, all of the gametes from a given parent contained the same allele. When gametes from two different parents combined, the resulting offspring, were heterozygous. In appearance, however, only the dominant trait was observed, because the F1 plants carried the dominant allele, which was expressed over the recessive allele. Slide 6 When Mendel’s F1 generations produced gametes, equal numbers of gametes contained dominant and recessive alleles. When these gametes combined, there would be an equal chance of each of the following allelic combinations in the offspring genotype: dominant-dominant, dominant-recessive, recessive-dominant, and recessiverecessive. However, the physical appearance of the F2 plants would be observed in the ratio three dominant to one recessive, because three of the possible allele combinations contain dominant alleles, one contains only recessive alleles. This three to one ratio is the ratio which Mendel observed. At this point, you should note that the physical appearance, or phenotype, of an organism, is determined by its genetic makeup, or genotype. In addition, different genotypes, such as the homozygous dominant and heterozygous conditions, can lead to the same phenotype. Slide 7 The results from Mendel’s first experiments led to the formation of Mendel’s first law, the law of segregation. The law of segregation states that during the formation of gametes, the alleles of a gene separate so that each gamete only receives one allele for each gene. Slide 8 Mendel’s second set of experiments involved measuring the heredity patterns of two characters at a time. This type of cross is called a dihybrid cross. For example, Mendel crossed plants that were true-breeding for seeds that were both round and yellow with plants true-breeding for green and wrinkled seeds. The results Mendel obtained were similar to those of his first experiments, although they were a little more difficult to interpret due to the inclusion of two characters rather than just one. In the F1 generation, all of the plants displayed both of the dominant traits found in the parental generation. In our example, the F1 generation all produced smooth, yellow seeds. In the F2 generation, Mendel always found that the traits in question appeared together in a 9:3:3:1 ratio. In other words, out of every sixteen F2 plants measured, nine had both dominant traits, six had only one of the dominant traits, and one displayed both recessive traits. Slide 9 To explain the results of his dihybrid crosses, Mendel proposed that not only did the alleles of each of the two genes segregate during gamete formation, as he had shown in his first experiment, but the two genes assorted independently of each other as well. For example, take a look at the F2 results in the illustration. If you add up only the round seeds compared to the wrinkled seeds, you will find 12 round and 4 wrinkled seeds. This, of course, is a three to one ratio, as is expected from Mendel’s first law. The same is true of the yellow and green seeds. However, when the characters of seed shape and color are considered together, the resulting 9:3:3:1 ratio can only be explained if the two genes for these characters have behaved independently of each other during gamete formation. When this occurs, gametes with all possible allele combinations (RY, Ry, rY, and ry) are formed with equal probability. When these gametes then combine to form offspring, the offspring are produced in a 9:3:3:1 phenotypic ratio. Slide 10 The results of Mendel’s dihybrid crosses led to the formulation of Mendel’s second law, called the law of independent assortment. This law states that during gamete formation, the alleles of different genes assort independently of one another. Due to this, any gamete may receive any combination of the alleles present in the organism. Recall that Mendel’s first law, the law of segregation, dealt with the segregation of different alleles of the same gene. The law of assortment describes the assortment of the alleles of different genes. Slide 11 Mendel’s work helped lay the foundation for modern genetics, and his discoveries and conclusions have largely stood the test of time. However, as we will see in the following lesson, there are a number of exceptions to Mendel’s second law, and a number of examples where the phenotype of an organism is not so clearly defined by strictly dominant and recessive alleles of genes. In fact, many researchers consider it amazing that all seven of the characters that Mendel chose for study displayed such clear patterns of dominant-recessive heredity. The exceptions to Mendel’s rules, of course, have presented some of the major challenges to geneticists since Mendel’s discoveries.