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Chapter 4-Mapping Eukaryote Chromosomes by Recombination1-Oct 3.docx

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Queen's University
BIOL 205
Kenton Ko

CH 4 MAPPING EUKARYOTE CHROMOSOMES BY RECOMBINATION Oct 3/2011 – Pg 129-134, 137-153, 163-165 - Why is a map of gene positions on the chromosomes so important? 1. Gene position is important when building complex genotypes required from experimental purposes or for commercial applications. 2. Knowing the position occupied by a gene provides a way of zeroing in on its structure and function 3. Genes present and arrangement on chromosomes are often slightly different in related species. o Ex. Chromosome 2 is split into 2 shorter chromosomes in great apes compared to humans. o Helps us deduce the evolutionary genetic mechanisms that caused these genomes to diverge. Chromosome maps - Chromosome maps tells us the arrangement of genes on chromosomes with gene positions Loci (loci) and the distances between the loci based on some kind of scale. - There are 2 types of maps currently used by geneticists: 1. Recombination-based maps: map the loci of genes identified by mutant phenotypes showing single gene inheritance 2. Physical maps show the genes as segments arranged along the long DNA molecule that constitutes a chromosome. 4.1 Diagnostics of Linkage Recombination maps - Recombination maps of chromosomes are assembled 2 or 3 genes at a time using linkage analysis. Linked genes - When 2 genes are linked, the loci of those genes are on the same chromosome and the alleles on any one homolog are physically joined by the DNA between them. Using Recombinant frequency to recognize linkage - Bateson and Punnett found a standard self of dihybrid F1, the F2 didn’t show 9:3:3:1 ratio. - Certain combinations of alleles showed up more often than expected, as though they were physically attached in some way. nd - Morgan also found a deviation from Mendel’s 2 Law by looking at genes that affected eye color (pr, purple, and pr+,red) and other gene affected wing length (vg, vestigial, and vg+, normal) o Pr/pr .vg/vg X pr+/  pr+/ F1 dihybrid o Testcross: pr+/pr,vg+vg X pr/ (tester male) o Morgan found 1339, 1195, 151, and 154 o These deviate from Mendelian prediction of 1:1:1:1 o The first 2 allele combinations are in the great majority, clearly indicating that they are associated or linked. - The recombinants are approximately equal in frequency or 10.7% frequency overall. - In the dihybrid, the alleleic conformation must have been: __pr+__vg+___ __pr___vg____ - Shows the tendency of linked alleles to be inherited as a package - In another testcross Morgan made, he found the F1 dihybrid must have been: __pr+__vg__ __pr__vg+__ - Dihybrid testcrosses follow the general pattern: 1. 2 equally frequent nonrecombinant classes totalling in excess of 50% 2. 2 equally frequent recombinant classes totalling less than 50% produced by 2 genes close together on a chromosome – linked. How Crossovers produce - When homologous chromosomes pair at meiosis, the chromosomes occasionally break and recombinants for linked exchange parts in a process called crossing over forming 2 new combinations called crossover genes products Crossing over - At meiosis, when duplicated homologous chromosomes pair with each other (2 dyads unite as a bivalent), a cross-shaped structure called a chiasma forms in between the 2 nonsister chromatids. Linkage Symbolism and Terminology - Linked genes may be present in one of two basic conformations. Cis conformation 1. Cis conformation: when alleles are present on the same homolog Trans conformation 2. Trans conformation: when alleles are on different homologs. - Note the following conventions: - Alleles on the same homolog have no punctuation between them - A slash symbolically separates the two homologs - Alleles are always written in the same order on each homolog - Genes known to be on different chromosomes (unlinked genes) are separated by a semicolon - Genes of unknown linkage are separated by a dot. 4.2 Mapping By Recombinant Frequency - The further apart the genes are, the more likely that a crossover will take place and the higher the proportion of recombinant products will be. - The proportion of recombinants is a clue to the distance separating two gene loci on a chromosome map. - A single crossover generates two reciprocal recombinant products which explains why the reciprocal recombinant classes are generally approximately equal in frequency and the two parental nonrecombinant types also must be equal in frequency. Map units - Sturtevant suggested that we can use the percentage of recombinants as a quantitative index of the linear distance between two genes on a genetic map , or linkage map. - In other meiotic divisions, there are no crossovers between genes, no recombinants result from these meioses. - The greater the chance of crossovers in the region between the genes, the greater the proportion of recombinants that would be produced. - The frequency of recombinants can be used to determine the map distance between the genes. Genetic Map Unit - Genetic map unit (m.u.) is the distance between genes for which one product of meiosis in 100 is recombinant. Recombinant Frequency o Ex. A recombinant frequency (RF) of 10.7% obtained by Morgan is defined as 10.7 mu. 1 map unit = recombinant frequency of 1%. - His analysis strongly suggested that genes are arranged in some linear order, making map distances additive o Ex. If the genetic distance between pr and vg loci in Drosophila is approximately 11 map units, we know that there will be 11% recombinants in the progeny from a testcross of a female dihybrid heterozygote in cis conformation. o These recombinants will consist of 2 reciprocal recombinants of equal frequency: 5.5% of each recombinant. o 89% will be nonrecombianant with 44.5% each frequency. Three Point Testcross - Crossing a trihybrid with a triply recessive tester. - Goal is to deduce whether 3 genes are linked and deduce their order and map distances between them. o Ex. The mutant alleles are v (vermillion), cv (crossveinless), and ct (cut wing edges) o Perform the following crosses: v+/ x v/v. cv+/cv+. Ct+/ct+ o We obtain v/ and cross with a tester male o From any trihybrid, only 8 gamete genotypes are possible and we can determine which genotypes are recombinant. o If the RF values are less than 50%, then the loci are linked. - Linkage maps merely map the loci in releation to one another, with the use of standard map units. We don’t know where the loci are on a chromosome or which specific chromosome they’re on. -
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