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Unformatted text preview: Mapping the genome: when chromosomes cross over Let's say you mate true-breeding Let' trueyellow pea/white flower & green pea/red flower lines... lines... P F1 F2 YYrr X yyRR YyRr ? "Independent assortment" predicts a 9:3:3:1 assortment" ratio of phenotypes. Using a Punnett square to predict genotypes & phenotypes assumes that the traits are controlled by two loci that are "unlinked" unlinked" (i.e., on different chromosomes). For pairs of gene loci that are on the same chromosome, this approach is inadequate. For 2 loci on the same chromochromosome... some... ...you might expect this. 1 Mitosis is cell division that yields new diploid cells with the full chromosome complement. Meiosis is cell division that yields "haploid" haploid" cells with half of the chromosome complement (i.e., gametes, with one member of each chromosome pair). When these unite, they form a diploid "zygote" with the zygote" usual chromosome complement (both members of each pair). 2 Major features: Prophase: condensation, n.m. breaks up (crossing over!) Metaphase: align at plate Anaphase: separation Telophase: Telophase: n.m. re-forms re- Crossing over can occur between all homologous sister chromatids. chromatids. "Odd-numbered" Odd- numbered" events produce recognizable recombinant gametes with respect to any two particular loci. 3 Chromosome map distances are approximately additive. I.e., for loci arranged on a chromosome in the order, A-B-C-D, the distance from A to D is equal to the distance from A to B + the distance from B to C + the distance from C to D. Whole chromosomes are mapped this way. Caveats: The maximum measurable distance is 50 cM (even though individual chromosomes are much longer than that). Individual distances >20 are suspect. Example: Assemble the following distances among loci into a map of the chromosome. Dist(A-C) = 17 cM Dist(ADist(A-B) = 3 cM Dist(ADist(B-C) = 20 cM Dist(BDist(B-D) = 5 cM Dist(B- Q) Why go to the trouble of recombining? It seems like it invites errors (misalignments resulting in unequal crossing-over). crossingA) It creates genotypic diversity (novel combinations of alleles) and even new alleles (when it occurs within loci)! These are important in evolution under changing conditions. "Unequal crossing-over" occurs when crossing- over" chromosomes misalign during recombination and exchange unequal-sized tips, resulting in unequalduplications and deletions of loci. 4 ...
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This note was uploaded on 04/18/2008 for the course BIOL_SCI 164 taught by Professor Walsh during the Spring '08 term at Northwestern.
- Spring '08