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Chapter 14 Mendel's Experiments

by: Saida Muktar

Chapter 14 Mendel's Experiments Bii

Marketplace > University of Maryland - Baltimore County > Bii > Chapter 14 Mendel s Experiments
Saida Muktar

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This is notes on chapter 14 that will be on the next exam
Intro to biological sciences
Dr. Omland
Class Notes
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This 5 page Class Notes was uploaded by Saida Muktar on Monday April 11, 2016. The Class Notes belongs to Bii at University of Maryland - Baltimore County taught by Dr. Omland in Winter 2016. Since its upload, it has received 45 views.

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Date Created: 04/11/16
14.1 Mendel’s Experimental System Heredity – transmission of traits from parents to offspring Trait – any characteristic of an individual ranging from height to the primary structure of a particular membrane protein - Mendel set out to address the most fundamental f all issues concerning heredity: What are the basic patterns in the transmission of traits from parents to offspring? o Two hypotheses:  Blending inheritance – claimed that the traits observed in a mother and father blend together to form the traits observed in their offspring. As a result, an offspring’s traits are intermediate between the mother’s and father’s traits.  Inheritance of acquired characters – claimed that traits present in parents are modified, through use, and passed on to their offspring in the modified form. o Blending inheritance contended that when black sheep and white sheep mate, their hereditary determinants will blend to form a new hereditary determinant for gray wool therefore offspring should be gray o Inheritance of acquired characters predicts that if giraffes extend their necks by straining to reach leaves high in the tops of trees, they subsequently produce longer-necked offspring. - Mendel chose garden pea because they are inexpensive and easy to grow from seed, have a relatively short generation time, and produce reasonably large number of seeds - Peas served as a model organism- a species that is used for research because it is practical and because conclusions drawn from studying it turn out to apply to many other species as well. - Two additional features of the pea made it possible for Mendel to design his experiments: individuals were available that differed in easily recognizable traits and he could control which parents were involved in mating. - Under normal conditions, garden peas self-fertilize – a flower’s pollen falls on the female reproductive organ of that same flower. - Mendel could prevent self-fertilization by removing the male reproductive organs from a flower before any pollen formed. Later he could transfer pollen from another pea plant to the target flower’s female reproductive organ with a brush. This type of mating is referred to as a cross-fertilization. Using this technique, Mendel could control the matings of his model organism - Mendel conducted his experiments on varieties of peas that differed in seven traits: seed shape, seed color, pod shape, pod color, flower color. Flower and pod position, and stem length. Phenotype – an individual’s observable traits. - In the first pea populations that Mendel studies, two distinct phenotypes existed for each of the seven traits. - Mendel began his work by obtaining individuals from pure lines. Pure line – consists of individuals that produce offspring identical to themselves when they are self-pollinated or crossed to another member of the pure-line population. - Mendel confirmed that individuals that germinated from his wrinkled seeds produced only wrinkled-seeded offspring when they mated to themselves or to another pure-line individual that germinated rom a wrinkled seed, and he confirmed the same was true for round seeds. - Suppose Mendel arranged matings between a pure line individual with round seeds and a pure line individual with wrinkled seeds. He knew that one parent carried a hereditary determinant for round seeds, while the other carried a hereditary determinant from wrinkled seeds, but each offspring from this mating would contain both types of hereditary determinants. Hybrids – offspring from matings between true-breeding parents that differ in on or more traits. 14.2 Mendel’s Experiments with a Single Trait - Mende’s first set of experiments consisted of crossing pure lines that differed in just one trait. - Mendel began his single-trait crosses by crossing individuals form round-seeded and wrinkled-seeded pure lines. The adults used in an initial experimental cross are the parental generation. Their progeny (offspring) are the F 1eneration. - In his first set crosses, Mendel took pollen from round-seeded plants and placed it on the female reproductive organs of plants from the wrinkled-seeded line. o The traits didn’t blend together to form an intermediate phenotype. Instead the round-seeded form appeared intact. o The genetic determinant for wrinkled seeds seemed to have disappeared. - Mendel planted the F se1ds and allowed the individuals to self- pollinate when they matured. Monohybrid cross – mating between parents that each carry two different genetic determinants for the same trait. - When he collected the seeds that were produced by many plants many resulting F2generation, he observed that 5474 were round and 1850 were wrinkled. The wrinkled seed shape had reappeared in the F 2 generation after disappearing completely in the F gen1ration. - Mendel invented some important terms to describe this result. o He designated wrinkled shape as a recessive trait relative to the round-seed trait.  Recessive allele – an allele that produces its phenotype only in homozygous form.  Phenotype disappears in heterozygous individuals o He referred to round seeds as dominant to the wrinkled-seed trait  Dominant allele –an allele that produces its phenotype in heterozygous and homozygous form.  Dominance does not imply high frequency or high fitness. - Reciprocal cross established that it does not matter whether the genetic dereminamnts for seed shape are located in the male or female parent. Particulate inheritance – hereditary determinants for traits do not blend together or become modified through use. Hereditary determinants maintain their integrity from generation to generation. Instead of blending together, they act as discrete entities or particles. Gene – a hereditary factor that influences a particular trait. Allele – a particular form of a gene The two alleles in a diploid may be the same or different. - Different alleles are responsible for the variation in the traits that Mendel studied. Genotype – the alleles found in a particular individual In diploids, the genotype lists two alleles of each gene in haploids, the genotype lists one allele of each gene. - To explain the reappearance of the recessive phenotype and the characteristic 3:1 ration of phenotypes in F 2ndividuals, Mendel reasoned that the two members of each gene pair must separate into different gamete cells during the formation of eggs and sperm. As a result, each gamete contains one allele of each gene. This idea is called the Principle of segregation. 14.4 Mendel’s Experiments with Two Traits - Mendel crossed a pure-line parent that produced round, yellow seeds with a pure-line parent that produced wrinkled, green seeds. According to his model, the F 1ffspring of this cross should be heterozygous for both genes. A mating between two such individuals – both heterozygous for two traits – is called a dihybrid cross - Hypothesis of independent assortment: Alleles of different genes do not stay together when gametes form. o The allele for seed shape and the allele for seed color originally present in each parent would separate from each other and transmitted independently. This hypothesis is called independent assortment because the two alleles would be sorted into gametes independently of each other. - Hypothesis of dependent assortment: Alleles of different genes stay together when gametes form. o The allele for seed shape and the allele for seed color originally present in each parent would be transmitted to gametes together. This hypothesis can be called dependent assortment because the transmission of one allele would depend on the transmission of another. - Mendel accepted the hypothesis that alleles of different gens are transmitted independent of one another. This result became known as the principle of independent assortment. Test cross – uses a parent that contributes only recessive alleles to its offspring and helps to determine the unknown genotype of the second parent. Testcrosses are useful because the genetic contribution of the homozygous recessive parent is known. As a result, a testcross allows experiments to test the genetic contribution of the other parent. 14.4 The Chromosome Theory of Inheritance - Meiosis Explains Mendel’s Principles - Meiosis explains the principle of segregation and the principle of independent assortment. o Principle of Segregation: Pairs of alleles are separated during meiosis in the formation of gametes.  The two members of a parent’s gene pair segregate into different gametes because homologous chromosomes separate during meiosis I. Chromosome theory of inheritance – states that Mendel’s rules can be explained by the independent alignment and separation of homologous chromosomes at meiosis I. o Principle of Independent Assortment: The genes for seed shape and seed color assort independently because they are located on different chromosomes. - Meiosis is responsible of the principle of independent assortment. The genes for different traits assort independently because nonhomologous chromosomes assort independently during meiosis. - If the alleles for different genes are located on different chromosomes, they assort independently of one another at meiosis I. this is the physical basis of Mendel’s principle of independent assortment. 14.5 Extending Mendel’s Rules - Linkage: What happens when genes are located on the same chromosome? Linkage – is the tendency of particular alleles of different genes to be inherited together. - Linkage is seen when genes are on the same chromosome - Because linked genes are located on the same chromosome, it is logical to predict that they should always be transmitted together during gamete formation. o Linked genes should violate the principle of independent assortment - Same chromosomes don’t always stay together. o Gametes with new combinations of alleles were generated when crossing over occurred during prophase of meiosis I in females. - Linked genes are inherited together unless crossing over occurs between them. When crossing over takes place, genetic recombination occurs.


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