Lecture 6 BIO 311D 1st Edition Outline of Last Lecture I Mapping the Distance Between Genes Using Recombination Data II Alterations of Chromosome Number or Structure cause Some Genetic Disorders III Human Disorders Due to Chromosomal Alterations IV Exceptions to Mendelian Genetics V Genomic Imprinting VI Inheritance of Organelle Genes Outline of Current Lecture I The Smallest Unit of Evolution II The Hardy Weinberg Equation III Genetic Variation IV Variation Within and Between Populations V Sources of Genetic Variation VI Altering Gene Number or Position Current Lecture The Smallest Unit of Evolution Microevolution is a change in allele frequencies in a population over generations Three mechanisms cause allele frequency change o Natural selection o Genetic drift when allele frequencies fluctuate o Gene flow movement of alleles due to different individuals mating Only natural selection causes adaptive evolution The original source of all genetic variation is Mutation The Hardy Weinberg equation Can be used to test whether a population has evolved The frequency of an allele in a population can be calculated o For diploid organisms the total number of alleles at a locus is the total number of individuals times 2 By convention if there are 2 alleles at a locus p and q are used to represent their frequencies o P is in reference to dominance o Q is used in reference to the recessive allele o P Q will always equal 1 For example consider a population of wildflowers that is incompletely dominant for color o 320 red flowers CRCR o 160 pink flowers CRCW o 20 white flowers CWCW Calculate the number of copies of each allele o CR 320 2 160 800 o CW 20 2 160 200 To calculate the frequency of each allele o p freq CR 800 800 200 0 8 o q freq CW 200 800 200 0 2 The sum of alleles is always 1 o 0 8 0 2 1 Remember that p 2 homozygous dominant 2pq heterozygous q 2 homozygous recessive 1 and p q 1 Example 1 in 1700 US Caucasian newborns have cystic fibrosis C for normal is dominant over c for cystic fibrosis 1 When counting the phenotypes in a population why is cc the most significant cc reveals all the individuals who are homozygous recessive and therefore have cystic fibrosis 2 What percent of the above population have cystic fibrosis cc or q 2 1 1700 0 0588 this is q 2 3 From the above numbers you should be able to calculate the expectant frequencies of all the following q 0 024 or 2 4 p 0 976 or 97 6 found by p q 1 and solve for p Genetic Variation Genetic variation makes evolution possible Genetic variation among individuals is caused by differences in genes or other DNA segments The phenotype is the product of inherited genotype and environmental influences Natural selection can only act on variation with a genetic component Variation Within a Population Both discrete and quantitative characters contribute to variation within a population Discrete characters can be classified on an either or basis Quantitative characters vary along a continuum within a population Genetic variation can be measured as gene variability or nucleotide variability For gene variability average heterozygosity measures the average percent of loci that are heterozygous in a population Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals Variation Between Populations Geographic variation differences between gene pools of separate populations Example Madeira is home to several isolated populations of mice o Chromosomal variation among populations is due to drift not natural selection Some examples of geographic variation occur as a cline a graded change in a trait along geographic axis o Example Mummichog fish vary in cold adaptive allele among a temperature gradient o This variation results from natural selection Sources of Genetic Variation New genes and alleles can arise by mutation or gene duplication Mutation a change in nucleotide sequence of DNA Only mutations in cells that produce gametes can be passed to offspring A point mutation is a change in one base in a gene The effects of point mutations can vary o Mutations in noncoding regions of DNA are often harmless o Mutations to genes can be neutral because of redundancy in the genetic code o Mutations that result in a change in protein production are often harmful o Mutations that result in a change in protein production can sometimes be beneficial Altering Gene Number or Position Chromosomal mutations that delete disrupt or rearrange many loci are typically harmful Duplication of small pieces of DNA increases genome size and is usefully less harmful Duplicated genes can take on new functions by further mutation An ancestral odor detecting gene has been duplicated many times humans have 1 000 copies of the gene mice have 1 300 The Hardy Weinberg Principle Describes a population that is not evolving If a population does not meet the criteria of the Hardy Weinberg principle it can be concluded that the population is evolving States that the frequencies of alleles and genotypes in a population remain constant from generation to generation In a given population where gametes contribute to the next generation randomly allele frequencies will not change Mendelian inheritance preserves genetic variation in a population
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