Study Guide Chapter 5 ? Linkage and Genetic Mapping in Eukaryotes Textbook: Genetics, 3rd Ed. By Robert Brooker GCD 3022 ? Fall 2010 Student Learning Objectives Upon completion of this chapter the student should be able to: Understand the concept of linkage and how it relates to patterns of inheritance. Understand, and be able to solve problems, of genetic mapping in diploid plants and animals. Study Outline Linkage and crossing over Crossing over may produce recombinant phenotypes Bateson and Punnett discovered two traits that did not assort independently Morgan provided evidence for the linkage of several X-linked genes and proposed that crossing over between X chromosomes can occur A chi square analysis can be used to distinguish between linkage and independent assortment Creighton and McClintock showed that crossing over produced new combinations of alleles and resulted in the exchange of segments between homologous chromosomes Crossing over occasionally occurs during mitosis Genetic mapping in plants and animals The frequency of recombination between two genes can be correlated with their map distance along a chromosome Alfred Sturtevant used the frequency of crossing over in dihybrid crosses to produce the first genetic map Trihybrid crosses can be used to determine the order and distance between linked genes Interference can influence the number of double crossovers that occur in a short region Key Terms An ascus is a sac that contains the cells of a tetrad or octad from Ascomycete fungi. The products of a single meiotic division are contained within this one sac. A bivalent is composed of two pairs of sister chromatids. Centimorgans (cM), or map units, are the units of distance. One map unit is equivalent to a 1% frequency of recombination. Crossing over is the phenomenon in which homologous chromosomes can exchange pieces with each other in diploid eukaryotic species. This event occurs during prophase of meiosis I. Dihybrid cross: When a geneticist follows the variants of two different traits in a cross A genetic linkage map maps the genes or other genetic sequences along a chromosome by analyzing the outcome of crosses. Genetic mapping seeks to determine the linear order and distance of separation among genes that are linked to each other along the same chromosome. It is based on the level of recombination that occurs in just one parent ? the heterozygote. Genetic recombination is the process in which alleles are assorted and passed to offspring in combinations that are different from the parents. Linkage has two related meanings: Linkage refers to the phenomenon that two or more genes may be located on the same chromosome. The genes are physically linked to each other, because each eukaryotic chromosome contains a single, continuous, linear molecule of DNA. Genes that are close together on the same chromosome tend to be transmitted as a unit. This indicates that linkage has an influence on inheritance patterns. Linkage groups are groups of genes on a chromosome that are linked together. The number of linkage groups equals the number of chromosome types. For example, human somatic cells have 46 chromosomes, which are composed of 22 types of autosomes that come in pairs plus one pair of sex chromosomes, the X and Y. therefore, humans have 22 autosomal linkage groups, an X chromosome linkage group, and a Y chromosome linkage group. The map distance is the number of recombinant offspring divided by the total number of offspring, multiplied by 100. Map units, or centiMorgans (cM), are the units of distance. One map unit is equivalent to a 1% frequency of recombination. Mitotic recombination is an event during which mitotic crossing over produces a pair of recombinant chromosomes that have a new combination of alleles. Nonparental, or recombinant, cells are haploid cells that contain a combination of alleles that are not found in the original chromosomes. Nonrecombinant, or parental, cells are haploid cells that contain the same combination of alleles that are found in the chromosomes of their parents in a testcross. An octad is a group of eight spores that are produced when, in some species of fungi, meiosis is followed by a mitotic division. An ordered tetrad (octrad) is when certain species of fungi produce a very tight ascus that prevents spores from randomly moving around. Parental, or nonrecombinant, cells are haploid cells that contain the same combination of alleles that are found in the chromosomes of their parents in a testcross. Positive interference is a phenomenon in which the occurrence of a crossover in one region of a chromosome decreases the probability that a second crossover will occur nearby. In other words, the first crossover interferes with the ability to form a second crossover in the immediate vicinity. Interference (I) is expressed as I=1-C Where C= Observed number of double crossovers Expected number of double crossovers Recombinant, or nonparental, cells are haploid cells that contain a combination of alleles that are not found in the original chromosomes. Spores are four haploid cells that are produced by certain species of fungi, which are the result of a diploid zygote that underwent meiosis. A testcross is used to determine if recombination has occurred during meiosis in the heterozygous parent. Most testcrosses are between an individual that is heterozygous for two or more genes and an individual that is recessive and homozygous for the same genes. A tetrad is the group of four spores produced by a diploid zygote that undergoes meiosis. Translocation occurs when one segment of a chromosome breaks off and becomes attached to a different chromosome. Trihybrid cross: When three traits are followed An unordered tetrad (octad) is created when, in some cases, the ascus provides enough space for the tetrads or octads of spores to randomly mix together. Study assignments: C5, C6, C10 E2, E3, E5, E7
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