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Chapter 11 Mendelian Genetics Copyright © 2010 Pearson Education Inc.
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Genetics is the study of the structure and function of genes. ◦ a.Early genetics focused on how traits are passed from parents to offspring (transmission genetics). ◦ b.Advances in biochemistry and molecular methods allow the study of the structure and function of genes at the molecular level (molecular genetics). Gregor Mendel (1822–1884) laid the foundation for our current understanding of heredity. Mendel did not know about chromosomes or genes, which were discovered after his lifetime
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Hereditary traits are under the control of genes (Mendel called them factors). Genotype is the genetic makeup of an organism, a description of the genes it contains. Phenotype is the characteristics that can be observed in an organism. Phenotype is determined by interaction of genes and environment. Genes provide potential, but environment determines whether that potential is realized
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Mendel began his work in 1854 with the garden pea, Pisum sativum, by crossbreeding plants with different characteristics. ◦ He reported his in 1865, but its significance was not realized until several decades later. He focused on well-defined traits one at a time, quantifying the offspring and analyzing the results mathematically. Garden peas are excellent for this type of research: ◦ they grow easily, ◦ produce large numbers of seeds quickly, ◦ routinely self-fertilize. ◦ experimental cross-fertilization is possible.
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First strains of peas were allowed to self-fertilize to be certain that the traits are unchanged in subsequent generations (true- breeding or pure-breeding strains). Inheritance of traits with only two distinct possibilities for phenotype The traits are: ◦ a.Flower/seed coat color (one gene controls both): *grey/purple vs. white/white. ◦ b.Seed color: *yellow vs. green. ◦ c.Seed shape: *green vs. yellow. ◦ d.Pod color: *green vs. yellow. ◦ e.Pod shape: *inflated vs. pinched. ◦ f.Stem height: *tall vs. short. ◦ g.Flower position: *axial vs. terminal. Dominant trait is with *
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a.Parental generation is the P generation. b.Progeny of P generation is the first filial generation, designated F 1. c.When F 1 interbreed, the second filial generation, F 2, is produced. d.Subsequent interbreeding produces F 3, F 4, and F 5 generations.
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A monohybrid cross involves true-breeding strains that differ in a single trait. To determine whether both parents contribute equally to the phenotype of a particular trait in offspring, a set of reciprocal crosses is performed. By convention, the female parent is given first. In Mendelian genetics, offspring of a monohybrid cross will exactly resemble only one of the parents. This is the principle of uniformity in F 1.
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Traits that disappear in the F 1 reappear in the F 2. The F 2 has a ratio of about one individual with the “reappearing” phenotype to three individuals with the phenotype of the F 1. Mendel reasoned that information to create the trait was present in the F 1 in the form of “factors,” now called genes.
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Each gene exists in alternative forms (alleles) that control a specific trait. ◦ True-breeding strains contain identical genes. The F 1 contain one of each, but since the trait is just like one of the parents rather than a mix, one (dominant) allele has masked expression of the other (recessive) one. By convention, letters may be used to designate alleles, with the dominant a capital letter (S) and the recessive in lowercase (s). ◦ Individuals with identical alleles (e.g., genotypes SS and ss) are homozygous for that gene. ◦ Individuals with different alleles (e.g., Ss) are heterozygous, because 1 ⁄ 2 of their gametes will contain one allele, and 1 ⁄ 2 the other.
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Diagrams of a smooth 3 wrinkled cross. The Punnett square is a diagram showing all possible gamete combinations of each parent. 3:1 ratio in the F 2 generation.
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After Mendel’s experiments for the seven different traits in garden peas he made these conclusions: ◦ a. Results of reciprocal crosses are always the same. ◦ b.The F 1 resembled only one of the parents. ◦ c.The trait missing in the F 1 reappeared in about 1 ⁄ 4 of the F 2 individuals.
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The first Mendelian law, the principle of segregation, states: ◦ “Recessive characters, which are masked in the F 1 from a cross between two true-breeding strains, reappear in a specific proportion in the F 2.” This is because alleles segregate during anaphase I of meiosis, and progeny are then produced by random combination of the gametes.
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Mendel observed that plants with the recessive phenotype are all true-breeding. When plants with the dominant phenotype are selfed, 1 ⁄ 3 are true-breeding, and 2 ⁄ 3 produce progeny with both phenotypes.
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Better approach to homozygous or heterozygous determination is testcross by crossing the individual with one that is homozygous recessive.
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Analyses of dihybrid cross (two pairs of traits) resulted in Mendel’s second law, the principle of independent assortment: ◦ factors for different traits assort independently of one another. This allows for new combinations of the traits in the offspring.
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If alleles assort independently, all possible phenotypes will be represented in the F 2, in a ratio of 9:3:3:1. ◦ If the F 1 are testcrossed, all types of offspring in a ratio of 1:1:1:1 will be produced. In the F 2 of a dihybrid cross there will be four phenotypic classes and nine genotypic classes.
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Mendel’s work was published in 1866 with little attention from the scientific community until about 1900, when Correns, deVries, and von Tschermark independently conducted experiments with similar results. In 1902 William Bateson, experimenting with fowl, showed that Mendelian principles apply in animals. He coined the terms genetics, zygote, F 1, F 2, and allelomorph (which was shortened to allele). W. L. Johannsen named Mendelian factors genes in 1909, from the Greek genos, meaning “birth.”
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W. Farabee in 1905 was the first to demonstrate Mendelian principles in humans, showing that brachydactyly is inherited as a simple dominant trait.
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The study of the phenotypic records of a family over several generations is pedigree analysis. The individual upon whom the study focuses is the propositus (male) or proposita (female). The symbols of pedigree analysis are summarized in,
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Recessive traits are usually the result of a mutation causing loss or modification of a gene product. ◦ Albinism is an example. Deleterious recessive alleles persist in the population because heterozygous individuals carry the allele without developing the phenotype and so are not at a selective disadvantage.
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Characteristics of recessive inheritance of a relatively rare trait: ◦ a.Parents of most affected individuals have normal phenotypes but are heterozygous. ◦ b.Mating of heterozygotes will produce 3 ⁄ 4 normal progeny and 1 ⁄ 4 with the recessive phenotype. ◦ c.If both parents have the recessive trait, all their progeny will usually also have the trait.
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Dominant trait is mutation causing a function to be gained because of an altered gene product capable of a new activity. ◦ Achondroplasia is an example. Dominant alleles produce a distinct phenotype when in a heterozygote with wild type allele. ◦ Due to the rarity of dominant mutant alleles causing recognizable traits, homozygous dominant individuals are very unusual.
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Characteristics of dominant inheritance of a relatively rare trait: ◦ a.Affected individuals have at least one affected parent. ◦ b.The trait is present in every generation. ◦ c.Offspring of an affected heterozygote will be 1 ⁄ 2 affected and 1 ⁄ 2 wild type. Other examples include: ◦ a.Autosomal dominant polycystic kidney disease (ADPKD). ◦ b.Brachydactyly. ◦ c.Marfan syndrome.
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