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slatkin4-5 - Bio 1B, Spring, 2008, Evolution section 1 of 4...

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Bio 1B, Spring, 2008, Evolution section 1 of 4 Updated 3/6/08 1:54 PM Lectures 4-5 4-5 Mutation and selection Reading:7 th edition 459-460, 464-468; 6 th edition 452, 456-459. 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. 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. 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
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This note was uploaded on 04/03/2008 for the course BIO 07853 taught by Professor Slatkin during the Spring '08 term at Berkeley.

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slatkin4-5 - Bio 1B, Spring, 2008, Evolution section 1 of 4...

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