Animal Genetics - MENDELU

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1 Mendel and his experiments Animal Genetics Gregor Johann Mendel ( ) was born in Heinzendorf, (nowadays in the Czech Republic). During the period in which Mendel developed his theory of heredity, he was a monk and a teacher of science in a public high school in what is now Brno, Czech Republic. He lived in a monastery, and it was there that he carried out his experiments on heredity. Mendel selected peas for his experiments for two reasons: firstly, he had access to varieties that differed in observable alternative characteristics, and secondly because his earlier studies of flower structure indicated that peas usually reproduce by self-pollination, in which pollen produced in a flower, is transferred to the stigma of the same flower. To produce hybrids by cross-pollination, he needed only to open the keel petal, remove the immature anthers before they had shed pollen, and dust the stigma with pollen taken from a flower on another plant. At the beginning of his experiments, he established true-breeding lines in which the plants produced only progeny like themselves when allowed to self-pollinate normally. These different lines (which bred true for flower colour, pod shape, or any of the other well-defined characters that Mendel had selected for investigation) provided the parents for subsequent hybridization. A hybrid is the offspring of a cross between inherently unlike parents. Mendel chose a common garden pea (Pisum sativum) for his first experiments in hybridisation. These plants exhibited what are now called Mendelian Traits - traits which occur in a very simple form. A simple trait in an organism is one which occurs either in one variation or another, with no in-between. Tab. :The seven traits that Mendel studied in the peas were: Trait: Dominant Expression: Recessive Expression: 1. Form of ripe seed Smooth/Round Wrinkled 2. Color of seed albumen Yellow Green 3. Color of seed coat/flower colour Grey/Purple White/White 4. Form of ripe pods Inflated Constricted

2 Trait: Dominant Expression: Recessive Expression: 5. Color of unripe pods Green Yellow 6. Position of flowers Axial Terminal 7. Length of stem Tall Dwarf Mendel s traits in animation Mendel called these characters paired differentiating characters and associated them with units, which were the cause of differences between them and which could be seen at first sight. It seems that this was the cardinal question because these units influenced those characteristic traits. These units are now known as genes, which became the basic term in genetic studies. Monohybrid crossing One pair of characters that Mendel studied was yellow versus green seeds. When pollen from a line of plants with green seeds was used to cross-pollinate plants from a line with yellow seeds, all of the produced hybrid seeds were round. He also performed the reciprocal cross, in which plants from the line with yellow seeds were used as the pollen parents and those from the line with green seeds as female parents. As before, all of the hybrid seeds were yellow. When the hybrid seeds from the yellow x green cross were allowed to undergo self-fertilization, the progeny were of two types (yellow and green), in definite, numerical proportions. Mendel counted 6022 seeds yellow and 2001 green, and noted that this ratio was approximately 3:1. The hybrid progeny produced by crossing the lines with yellow and green seeds constitute the F 1 generation, and all of the seeds were yellow. The progeny produced by self-fertilization of the F 1 generation constitute the F 2 generation, and the F 2 progeny appeared in the proportions 3 yellow : 1 green. The principal observations of his experiments were: The F 1 hybrids possessed only one of the parental traits.

3 In the F 2 generation, both parental traits were present. The trait that appeared in the F 1 generation was always present in the F 2 about three times as frequently as the alternative trait. Tab.: Numbers and ratios in F 2 generation of monohybrid crosses: Trait No. of individuals No. of individuals with dominant trait No. of individuals with recessive trait Seed shape : 1 Cotyledon colour : 1 Flower colour : 1 Pod shape : 1 Pod colour : 1 Flower position : 1 Stem length : 1 Phenotypic ratio in F2 Mendel mentioned the following: Each of these seven hybrid traits equals to one of both essential traits in such a perfect way that the other can be either overlooked or is so similar to the other that they cannot be securely distinguished. This fact is very important for the determination and classification of those forms in which the progeny of hybrids is produced. Principle of segregation The principle of segregation describes how pairs of gene variants are separated into reproductive cells.

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5 Mendel discovered that the traits in the offspring of his crosses did not always match the traits in the parental plants. This mean that the pair of alleles encoding the traits in each parental plant had separated or segregated from one another during the formation of the reproductive cells.

6 The most revolutionary on Mendel s ideas was that of a free combining ability of traits and their material principles, which was scientifically demonstrated as late as in the 20 th century in studies describing meiotic division by which sex cells (gametes) were produced. Dihybrid crossing Based on the concept of segregation, he predicted that traits must sort into gametes separately. By extrapolating from his earlier data, Mendel also predicted that the inheritance of one characteristic did not affect the inheritance of a different characteristic. Mendel tested idea of trait independence with more complex crosses. First, he generated plants that were purebred for two characteristics, such as seed color (yellow and green) and seed shape (round and wrinkled). These plants would serve as the P generation for the experiment. Principle of independet assortment The principle of independent assortment describes how different genes independently separate from one another when reproductive cells develop. Mendel was performing dihybrid crosses, which are crosses between organisms that differ with regard to two traits. He discovered that the combinations of traits in the offspring of his crosses did not always match the combinations of traits in the parental organisms. Mendel crossed parental plant that differed in two or three pairs of alleles affecting different characters. For example, plants from a true-breeding line having round and yellow seeds were crossed with plants from a line having wrinkled and green seeds. The F 1 seeds from this cross were round and yellow; they were dihybrid (hybrid for both characteristics).

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8 We now know that this independent assortment of genes occurs during meiosis in eukaryotes.

9 Through carefully designed hybridization experiments with the garden pea and numerical analyses of progeny, Gregor Mendel deduced fundamental principles of transmission genetics. 1. The principle of segregation states that alleles of a gene pair segregate from each other during gamete formation so that gametes ha ve only one member of a gene pair. 2. The principle of independent assortment states that alleles of different genes segregate, or assort, independently of each other, and recombine at random at fertilization. In other words, the segregation of one gene pair does not interfere with or influence the segregation of other gene pairs. 3. Mendel also discovered another principle involving gene function, referred to as dominance. 4. The blending theory of inheritance was discounted. 5. Acquired traits are not inherited. Mendel's discoveries went essentially unnoticed until 35 years later.

10 1. 2. ln 1900, three scientists (Correns, devries, and von Tschermak-Seysenegg) independently rediscovered Mendel's laws. The discipline of genetics was bom in 1900 when William Bateson championed the cause of Mendelian genetics and introduced it to the English-speaking world. Conclusion Discrete units called genes control the appearance of inherited traits. Genes come in altemative forms called alleles that are responsible for the expression of different forms of a trait. Body cells of sexually reproducing organisms carry two copies of each gene. When the two copies of a gene are the same allele, the individua! is homozygous for that gene. When the two copies of a gene are different alleles, the individua! is heterozygous for that gene. The genotype is a description of the allelic combination of the two copies of a gene present in an individua!. The phenotype is the observable form of the trait that the individua! expresses. A cross between two parental lines (P) that are purebreeding for altemative alleles of a gene will produce a first filial (F 1 ) generation of hybrids that are heterozygous. The phenotype expressed by these hybrids is determined by the dominant allele of the pair, and this phenotype is the same as that ex.pressed by individuals bomozygous for the dominant allele. The phenotype associated with the recessive allele will reappear only in the F 2 generation in individuals homozygous for this allele. In crosses between F 1 heterozygotes, the dominant and recessive phenotypes will appear in the F 2 generation in a ratio of 3:1. The two copies of each gene segregate during the formation of gametes. As a result, each egg and each sperm or pollen grain contains only one copy, and thus, only one allele, of each gene. Male and female gametes unite at random at fertilization. Mendel described this process as the principle of segregation. The segregation of alleles of any one gene is independent of the segregation of the alleles of other genes. Mendel described this process as the principle of independent assortment. According to this principle, crosses between Aa Bb F 1 dihybrids will generate F 2 pro geny with a phenotypic ratio of 9 (A- B-) : 3 (A- bb) : 3 (aa B-) : 1 (aa bb). The document was created: :02:44 Source: