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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. Background work by scientists:. Mendel: crossed pea plants and determined the 9:3:3:1 ratio in dihybrid crosses. Concluded: 2 alleles/trait and alleles assort independently

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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. Background work by scientists: • Mendel: crossed pea plants and determined the 9:3:3:1 ratio in dihybrid crosses. • Concluded: 2 alleles/trait and alleles assort independently • Punnett and Bateson: crossed pea plants and found that the dihybrid ratios differed from 9:3:3:1 • Concluded: some traits must be linked

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

  4. Sturtevant’s hypothesis: • The frequency of cross-overs during meiosis is related to the distance between genes: the greater the distance, the greater the frequency of crossover • S. experiments: studied linked traits in fruit flies, used crossover frequencies to make linkage maps • Making a linkage map: • Crossover frequencies are converted to map units • Can find the relative location of genes

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

  6. 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.

  7. 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

  8. IMPORTANT: 1% crossover = 1 map unit • By knowing all crossover frequencies, the relative location of the genes can be determined. • (This is NOT an actual map of the physical distance between genes.)

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