Ro_Lecture5

Ro_Lecture5 - Lecture 5 1 Linked genes recombination and...

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1 Lecture 5
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2 Linked genes, recombination, and chromosomal mapping Mendel's Law of Independent Assortment is a consequence of the fact that chromosomes segregate independently in meiosis These results are readily explained by the two alternative ways the chromosomes can line up on the metaphase plate during meiosis I: Take two individuals One heterozygous and one homozygous AaBb x aabb 25% 25% 25% 25%
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3 MeiosisI These results are readily explained by the two alternative ways the chromosomes can line up on the metaphase plate during meiosis I: A a B b OR A a b B AB ab Ab aB Because the A and B genes assort independently, AaBb dihybrids constructed from different parental genotypes will behave the same. AABBxaabb AAbbxaaBB AaBbxAaBb
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4 AaBb x aabb 25% 25% 25% 25% 25% 25% 25% 25% AABB x aabb AAbb x aaBB AaBb x aabb A a B b
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5 Linkage? A dihybrid cross involving the genes A and C produced the following results: · A= Tall a= short · C= Cream c = white Cross I: Cross II: Tall, Cream x short, white Tall, white x short, Cream AACC aacc AAcc aaCC Tall, Cream AaCc Tall, Cream AaCc X X short, white aacc short, white aacc 50% Tall, Cream 50% Tall, white 50% short, white 50% short, Cream
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6 In these crosses, independent assortment is not occurring. For example in the first cross, the alleles Tall and Cream behave as if they are linked to one another. Similarly in the second cross the alleles Tall and white appear as if they are linked to one another. These results are readily explained if the genes A and C lie next to one another on a chromosome:
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7 Cross I: A= Tall a= short C= Cream c = white A-C A-C a-c a-c A-C a-c a-c a-c a-c A-C a-c A-C a-c a-c a-c Tall cream Short white
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8 Cross II: A= Tall a= short C= Cream c = white A-c A-c a-C a-C A-c a-C a-c a-c a-c A-c a-C A-c a-c a-C a-c Tall white Short cream
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9 Coupling/repulsion Results like these led Morgan to suggest that the A and C genes are located on the same pair of homologous chromosomes. Therefore when the A and C alleles are introduced from one
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Ro_Lecture5 - Lecture 5 1 Linked genes recombination and...

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