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huelsenbeck 4

huelsenbeck 4 - Lecture 4 Forces causing gene frequency...

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Lecture 4 Friday, March 6, 2009 Forces causing gene frequency change Random mating does not cause allele frequencies to change, but other forces do. Mutation creates new alleles but mutation rates are so low that that mutation has little effect on the frequencies of alleles already present in a population. Alleles frequencies change because of the combined effects of natural selection, genetic drift, gene flow and recombination. Mutation A mutation is the result of an error in DNA replication: A change in a single nucleotide is called a point mutation. In higher plants and animals, the probability of a point mutation (i. e. the mutation rate) is very low, close to 1 in a billion (10 –9 ). Rates of point mutations are higher in bacteria and higher still in viruses. Other types of mutations occur : small or large pieces of chromosomes can be deleted or duplicated. A duplication can create a second copy of a gene. Whole chromosomes can be duplicated : In humans, individuals born with 3 copies of chromosome 21 (trisomy 21) have Down’s syndrome. The risk of trisomy 21 in the US population is between 1/650 and 1/1000; the risk increases with maternal age 1 . Whole genomes can be duplicated : If there is no reduction division during meiosis, diploid gametes are produced. A diploid gamete combined with a haploid gamete creates a triploid zygote. Bananas and many other domesticated plants are triploid. Natural selection Fitness: If individuals with different genotypes differ in their chances of survival and reproduction, then there are differences in fitness that cause allele frequencies to change. Average fitnesses of different genotypes may depend on the environment. For example, individuals with defective alleles of the PAH gene who eat a normal diet die at an early age because of the accumulation of phenylalanine. They suffer from phenylketonuria (PKU). If phenylalanine is removed from the diet, there is almost no reduction in survival rate. 1 Information about trisomy 21 and other inherited diseases and conditions in humans can be found from OMIM (Online Mendelian Inheritance in Man), http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM.
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Directional selection: Directional selection occurs when one allele results in higher rates of survival and reproduction. For example, if there are two alleles A and a, and AA individuals have a higher fitness than Aa individuals who have a higher fitness than aa individuals, then A is the advantageous allele and a is the deleterious allele. If a population initially contains only aa individuals and an advantageous allele A is created by mutation, then the frequency of A will increase every generation because of natural selection. Eventually A will be substituted for a . In the 1920s, population geneticists showed that natural selection can cause
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huelsenbeck 4 - Lecture 4 Forces causing gene frequency...

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