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Gene Linkage

Gene Linkage. Chapter 19.2 Page 635-641. Gene Linkage and Crossover. Back to Morgan and the fruit flies first Morgan crossed wild-type body colour ( AA ) and straight wings ( BB ) with black body colour ( aa ) and curved wings ( bb ) AABB x aabb

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Gene Linkage

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  1. Gene Linkage Chapter 19.2 Page 635-641

  2. Gene Linkage and Crossover Back to Morgan and the fruit flies • first Morgan crossed wild-type body colour (AA) and straight wings (BB) with black body colour (aa) and curved wings (bb) AABB x aabb • the F1 generation is heterozygous for both traits AaBb • Morgan then crossed the F1 generation and expected the Mendelian ratio of 9:3:3:1 for a dihybrid cross. • instead all the individuals with wild-type body colour had straight wings and all those with black body colour had curved wings.

  3. Gene Linkage and Crossover • Morgan concluded that the two genes: • did not undergo independent segregation • for this to happen genes would have to be on the same chromosome • the genes are linked

  4. Gene Linkage and Crossover • because the genes are linked the two gametes form an individual that is heterozygous for both traits. • Morgan predicted that crossing the F1 generation would produce an F2 generation what would have a 3:1 phenotypic ratio (3 flies with wildtype body type and straight wings)

  5. Linked genes

  6. Crossing over • Morgan observed that in a small number of dihybrid crosses the offspring had different combinations of traits than the parents Data:

  7. Crossing Over • the new combinations of alleles that Morgan observed came about by DNA crossing over during meiosis.

  8. Crossing Over and Chromosome Mapping • there are groups of linked genes on a chromosome. • these are called a linkage group • particular genes are always found on the same location (locus) on a chromosome. • Morgan showed that the frequency of crossovers between any two genes in a linkage group is always the same. • the frequency of crossing over between any two genes can be stated as a percent.

  9. crossover percentage = (number of recombinations/total number of offspring) x 100% using the previous table’s data: crossover % = 18/400 x 100% crossover % = 4.5% • geneticists use this number to say the two alleles are 4.5 map units apart.

  10. probability = (genotype looking for)/ (total number of possible genotypes) probability = (phenotype looking for)/ (total number of possible phenotype)

  11. Homework! • Review sample question page 638 • Complete Lab Exercise 19.B page 639-640 – Due next class!

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