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DeNegre+Lab+5_linkage

DeNegre+Lab+5_linkage - Lab 5 Linkage Recombination Genes...

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Lab 5 - Linkage & Recombination
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Genes Can Be Linked There are 20,000-25,000 genes in the human genome – so, it can’t be that each gene is on its own chromosome and segregates independently. In many cases, genes travel together in particular allelic sets. [We call this “linkage.”]
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Independent Assortment (& Segregation) Because of the way homologous chromosomes line up on the metaphase plate, we expect 4 different types of gametes in equal proportions : [Meiosis in an A/a;B/b individual]
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Another way to say this is that ½ the gametes show the “parental” configuration, while the other ½ show a “recombinant” (non-parental) configuration.
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What if A and B are close together on the same chromosome? In the absence of crossing over, linked genes stay together. Crossing over can often separate linked genes, however. [“Complete Linkage”] [“Incomplete linkage”]
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How do we know if genes are “linked” or “independent”? Independent assortment gives us 4 types of gametes (2 parental and 2 recombinant) in EQUAL FREQUENCIES (1/4:1/4:1/4:1/4) [heterozygote] Complete linkage gives us 2 types of gametes (both parental) in EQUAL FREQUENCIES (1/2:1/2) With incomplete linkage… it depends on how close the linked genes are.
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Linked Genes If the alleles of the A and B genes get mixed and matched 10% of the time, we say that their loci are 10 “centiMorgans” (cM) apart. [Note: “map distance” = frequency of recombination. It doesn’t necessarily have anything to do with the actual, physical distance between the loci.]
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Recombination Frequency * The number of recombinant offspring, divided by the total number of offspring (expressed as a percent).
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Coupling and Repulsion With linkage, it’s not always the case that one
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