Chapter 7_post

Genetics: A Conceptual Approach

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Chapter 7 Linkage, Recombination & Eukaryotic Gene Mapping Dr. Ed Otto George Mason University
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Key objectives Understand differences in the patterns of inheritance of unlinked and linked genes Understand how these patterns relate to chromosome behavior during meiosis Be able to test for independent assortment using the chi square contingency test Be able to construct genetic maps using two-point and three-point testcrosses Understand physical mapping methods
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Mendel’s Principles and the Inheritance of Unlinked Genes
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Mendel’s principles Segregation : each diploid organism possesses two alleles of a gene that separate in meiosis, with one allele going to each gamete. Independent Assortment : two alleles of one gene separate and are inherited independently from alleles at other loci
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Testcross- genes unlinked Each gamete receives one of the two alleles for each gene ( segregation ) Alleles of gene M separate independently from alleles of gene D ( independent assortment ) M = normal leaves m = mottled leaves D = tall plant d = dwarf plant Dominance: M > m D > d
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Testcross- genes unlinked Ratio of progeny is 1:1:1:1 Progeny are 50% nonrecombinant (like parents) and 50% recombinant (unlike parents)
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M M m m D D d d M M m m D D d d M M m m d d D D M M m m d d D D M M D D ¼ MD m m d d ¼ md M M d d m m D D ¼ Md ¼ mD Meiosis I Meiosis II or with equal probability - each gamete receives one allele for each gene (segregation) - alignment & separation of each pair of homologous chromosomes is independent of other pairs (independent assortment)
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Unlinked genes Genes that reside on different chromosomes assort independently and are referred to as unlinked Unlinked genes belong to different linkage groups
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Inheritance of Linked Genes
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Historical perspective All of the characteristics examined by Mendel displayed independent assortment How could many genes be carried on a limited number of chromosomes and yet sort independently? This apparent inconsistency disappeared as biologists began finding genetic characteristics that did not assort independently
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Linked genes Genes located close together on the same chromosome are called Linked Linked genes do not assort independently, but travel together during meiosis and arrive at the same destination (the same gamete) Linked genes belong to the same linkage group
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Notation for crosses with linkage For unlinked genes, we previously wrote genotypes as: Mm Dd For linked genes, it is necessary to write out the specific alleles as they are arranged on each homologous chromosome M D m d • M and D alleles are on one homologous chromosome; m and d alleles on the other • 2 alleles at a locus are always located on differ homologous chromosomes and must lie on opposite sides of the lines M D m d M D m d • Notation for linked genes can be further simplified: MD / md = =
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Testcross- genes linked Only nonrecombinant gametes are formed Progeny ratio is 1:1
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This note was uploaded on 05/06/2010 for the course BIO 311 taught by Professor Otto during the Spring '10 term at George Mason.

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Chapter 7_post - Chapter 7 Linkage Recombination Eukaryotic Gene Mapping Dr Ed Otto George Mason University Key objectives Understand differences

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