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MIT6_047f08_lec11_note11

MIT6_047f08_lec11_note11 - MIT OpenCourseWare...

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MIT OpenCourseWare http://ocw.mit.edu 6.047 / 6.878 Computational Biology: Genomes, Networks, Evolution Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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6.047 LECTURE 11, MOLECULAR EVOLUTION/COALESCENCE/SELECTION/KAKS OCTOBER 09, 2008 1. Introduction Evolution is the shaping of phenotypic and genotypic variation by natural selec- tion. The relative importance of selection and mutation has been a long standing question in population genetics. And, we’d like to understand the relative impor- tance of each of these forces. In particular, we’d like to know which genes are undergoing active selection. neutrality tests are a key tool for addressing this ques- tion. There are many different evolutionary forces that might cause deviations from neutrality such as differential mutation rates, recombination, population structure, drift in addition selection and others. Therefore, it is easier to develop test of neu- trality rather than directly searching for signatures of selection. From a historical perspective, most of these ideas were developed theoretically in the last century. And only in the last few decades that we were able to gather data to directly test these theories. 1983 is the first time that we have molecular polymorphism data. First, we need to understand the neutral model, focusing on inheritance alone. Historically, this area of study dates back to Darwin’s time. Scientists in the 1860s believed in blending inheritance which stated that each organ was determined by a different gemmule . In this model, children inherit a blending of their parents’ corresponding gemmules , so if mom had a “yellow” gemmule and dad had a “blue” gemmule, then baby would inherit a “greenish” gemmule. As Darwin’s critics pointed out, this model predicts that everyone’s gemmules blend and blend until all gemmules are a drab shade of gray, losing all genetic diversity. Specifically, Fleeming Jenkins in 1867 showed that the total genetic variation will be halved assuming this mode of inheritance. However, Mendel had developed a theory particulate inheritance around the same time but was not widely recognized. Only in the beginning of 20th cen- tury, researchers appreciated the importance of his results. Mendel’s more accurate model, developed as a result of his famous study of pea plants, states that each trait is determined by two corresponding alleles , which together determine a per- son’s phenotype . Offspring receive one allele from each parent, selected randomly from the parent’s two copies. The allele model is quite close to what actually happens when gametes pair during meiosis, and correctly predicts the observed phenomena of dominance and recessivity. These ideas are summarized as the Law of Segregation and the Law of Independent Assortment.
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