Lecture 6 BIO 311D 1st EditionOutline of Last Lecture I. Mapping the Distance Between Genes Using Recombination DataII. Alterations of Chromosome Number or Structure cause Some Genetic DisordersIII. Human Disorders Due to Chromosomal AlterationsIV. Exceptions to Mendelian GeneticsV. Genomic ImprintingVI. Inheritance of Organelle GenesOutline of Current Lecture I. The Smallest Unit of EvolutionII. The Hardy-Weinberg EquationIII. Genetic VariationIV. Variation Within and Between PopulationsV. Sources of Genetic VariationVI. Altering Gene Number or PositionCurrent LectureThe Smallest Unit of Evolution:- Microevolution is a change in allele frequencies in a population over generations- Three mechanisms cause allele frequency change:o Natural selectiono Genetic drift: when allele frequencies fluctuateo Gene flow: movement of alleles due to different individuals mating- Only natural selection causes adaptive evolutionThe original source of all genetic variation is: MutationThe Hardy-Weinberg equation:- Can be used to test whether a population has evolved- The frequency of an allele in a population can be calculatedo 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 frequencieso P is in reference to dominanceo Q is used in reference to the recessive alleleo 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 = 800o CW = (20 ´ 2) + 160 = 200- To calculate the frequency of each allele:o p = freq CR = 800 / (800 + 200) = 0.8o q = freq CW = 200 / (800 + 200) = 0.2- The sum of alleles is always 1o 0.8 + 0.2 = 1Remember that: p^2 (homozygous dominant) + 2pq (heterozygous) + q^2 (homozygous recessive)= 1 and p + q =1Example: 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 componentVariation 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 individualsVariation Between Populations:- Geographic variation: differences between gene pools of separate populations - Example: Madeira is home to several isolated populations of miceo 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 gradiento This variation results from natural selectionSources 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 harmlesso Mutations to genes can be neutral because of redundancy in the genetic codeo Mutations that result in a change in protein production are often harmfulo Mutations that result in a change in protein production can sometimes be beneficialAltering 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 1,300The 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
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