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KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

KEY CONCEPT Genes can be mapped to specific locations on chromosomes. Chromosomes and Recombination Many biologists refused to believe Mendel’s principles of heredity, IE> independent assortment & segregation

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KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

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  1. KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

  2. Chromosomes and Recombination • Many biologists refused to believe Mendel’s principles of heredity, IE> independent assortment & segregation • 1900- Three European scientists rediscover Mendel’s papers and separately confirmed his conclusions • In 1880-1890’s new methods for staining chromosomes make them visible under the microscope • These new methods linked Mendel’s factors (or genes) to chromosomes • Chromosome Theory of Heredity • 1903- Walter S. Sutton of Columbia University published two papers outlining a chromosome theory of heredity. He recognized similarities between Mendel’s factors and chromosomes, IE> • Chromosome theory of heredity- the material of inheritance is carried by the chromosomes.

  3. Sutton’s conclusions- • particular genes are located on particular chromosomes (this conclusion also made by German scientist Theodor Boveri) • many different genes on a chromosome, since thousands of traits but not thousands of chromosomes • some chromosomes are related to a number of traits so all traits represented by a single chromosome must be inherited together

  4. Linked Genes • Bateson & Punnett (1906)- sweet pea genetic experiment between purebred parents did not show expected ratio in F2 offspring • P1-PPLL x ppll, where: • P = purple color • p = red • L = long pollen shape • l = round pollen • F1– Dominant Phenotype • F2-They expected  9 (purple/long): 3 (purple/round): 3 (red/long): 1 (red/round) ratio, the classic 9:3:3:1 phenotype

  5. Their numbers didn’t come out right • They realized that genes for flower color and pollen shape were not assorting independently, but instead appeared together as they had in the parental generation • They concluded that certain genes were coupled or connected somehow.

  6. Mutant Wild type Gene linkage was explained through fruit flies. • Morgan found that linked traits are on the same chromosome. • Chromosomes, not genes, assort independently during meiosis.

  7. Thomas Hunt Morgan- zoologist found similar results using the fruit fly Drasophila, he concluded that these results are explained if the genes were carried on the same chromosome • Linked genes- genes that are on the same chromosome. They tend to not assort independently and are thus inherited together. • Genetic Recombination- • A few plants had purple/round & a few had red/long. This was due to crossing over, or the exchange of genes between homologous chromosomes during meiosis. • Genetic recombination- the shuffling of genes into new combinations by crossing over. • Recombinant- an organism or a chromosome with a recombined set of genes • Gene Mapping- is locating genes on chromosomes. It is accomplished by working with two traits at a time. Geneticists compare the # of offspring w/ recombinant genes with the # of offspring in which crossing over did not occur. From this information the geneticist can deduce how far apart the genes are on the chromosome. By repeating this experiment with different genes a genetic map can eventually be produced.

  8. Genes on Chromosomes • Thomas Hunt Morgan (1910)- further evidence of validity of the chromosome theory of inheritance Using fruit fly Drasophila, which had several characteristics making it a good subject for genetic experimentation: • Easy to feed • Easy to maintain • Produce new generations in ~ 2 weeks, so easy to accumulate information • Only 4 pairs of chromosomes • Easily observable by microscope • Sex chromosomes- one of the 4 chromosomal pairs that determines the sex of the fruit fly • Sex chromosomes- discovered in 1905 by Nettie Stevens (colleague of Morgan & Boveri) in a type of beetle • XX = female • XY = male • Morgan’s Experiments • Normal fruit fly eye color is red • Morgan found a white eyed male, this eye color variation is an example of a gene/chromosome spontaneously changing or mutating • Mutation- a spontaneous change in a gene or chromosome • Mutant phenotype- the phenotype produced by a mutation • Morgan crossed red eye female (XRXR) w/ white eye male (XrY) • F1 off spring all red eyed- conclusion white eye allele recessive to red eye allele

  9. Chromosomes exchange homologous genes during meiosis. • Linked genes are not inherited together every time.

  10. Linkage maps estimate distances between genes. • The closer together two genes are, the more likely they will be inherited together. • Cross-over frequencies are related to distances between genes. • Linkage maps show the relative locations of genes.

  11. Cross-over frequencies can be converted into map units. • gene A and gene B cross over 6.0 percent of the time • gene B and gene C cross over 12.5 percent of the time • gene A and gene C cross over 18.5 percent of the time

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