Lecture 19b - Lecture 19 Population Genetics The phenomena...

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Population Genetics Lecture 19
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Organisms interact with their environments and with one another in populations , specifically for our purposes, as groups of individuals that mate with one another to produce the next generation. Population genetics is the study of the allele frequency distribution and change under the influence of the evolutionary processes such as mutation and recombination, gene flow (immigration of genes), natural selection… The phenomena that drive evolution We will consider: The effect of random mating on different genotypes in a population The introduction of new genetic variation (alleles) by mutation the production of new combinations of alleles by recombination changes in population composition due to natural selection (fecundity and survival of offspring) genetic drift in allele frequencies from generation to generation.
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Observing variation Characters employed in population genetics must have a simple relation between phenotype and genotype such that variation can be shown to be the result of different allelic forms of a single gene. An example are the phenotypes of red blood cells. This level of phenotype is attractive because it is insensitive to environmental factors . Furthermore, different phenotypes can be shown to be the result of different allelic forms of a single gene. The MN component of the human blood group system MNS shows a simple one to one correspondence of genotype to phenotype. There is no dominance among the alleles.
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More than 40 antigenic specificities are known to occur on human blood cells. Variation in genotypic frequencies between individuals within populations as well as between populations
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For the MN system genotypic and allelic frequencies are given in the table… typically allele frequencies are employed in population genetics. The allele frequency is simply the proportion of that allelic form of the gene among all the copies of that gene in the population, where each individual diploid organism contributes two alleles for each gene . Homozygotes for a given allele have 2 copies for that allele Heterozygotes have only one copy The frequency for an allele is the frequency of homozygotes plus half of the frequency of heterozygotes p = 0.835 + ½ (0.156) = 0.913 q = 0.009 + ½ (0.156) = 0.087 1.000 p + q = 1.0
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In the ABO system there are two dominant alleles ( I A and I B ) and one recessive allele ( i). Because the heterozygotes of that carry the i allele will look like the homozygote dominant, additional analysis is required (sequence/examine pedigrees/conduct crosses). Variation can be observed at various levels from the external morphology down to the amino acid sequences of proteins; genetic variation ( polymorphism ) can be characterized directly from DNA sequence. If there is one form of a gene or a single phenotype for a character that predominates over all other rare types it is said to be wild type . If two or more forms are common it is not possible to designate a wild type allele.
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Lecture 19b - Lecture 19 Population Genetics The phenomena...

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