09.27.11 - Lecture 7. Three point mapping, centromere...

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Unformatted text preview: Lecture 7. Three point mapping, centromere mapping, and recombination. Readings: Chapt 4, 132-137 142-152 Concepts 1.Genetic maps are assembled by comparing the segregation of multiple genes at one time, but the best estimate for the genetic distance of two genes is based solely on comparing adjacent pairs of genes. 2.Using tetrad analysis the distance between a gene and a centromere can be determined. 3.The maximum amount of recombination between any pair of linked genes is 50%. For linked genes, recombinant frequencies are less than 50 percent Higher r yields more recombinant types Higher r yields more recombinant types However, any given tetrad has only recombinant gametes when crossing over occurs. Therefore, r is always < 0.5 p , Nb Ab AB ab Lets look at that data more closely recombination between w and y = 0.5% = 0.5 cM recombination between w and m = 34.5% = 34.5 cM Distance is really close Distance is really far Ordering three genes (three-point mapping) takes advantage of the fact that multiple crossovers can occur per chromosome Double crossover has a much lower rate than single crossover in general. Single crossover between A and B: 10% Single crossover between B and C: 20% Double crossover: 2% if the two events are independent. Ordering three genes (three-point mapping) must take into account that multiple crossovers can occur per chromosome Double crossover has a lower rate than single crossover in general. Single crossover between A and B: 10% Single crossover between B and C: 20% Double crossover: 10% x 20%=2% if the two events are independent. Requirements for three-point mapping 1. The organism producing the crossover gametes must be heterozygous at all loci under consideration. 2. The cross must be constructed in such a way that the genotype of the gamete can be determined from phenotype of offspring. 3. A large sample is required to obtain all crossover classes. Three-point mapping Steps for solving a three point mapping problem. Scenario: we have three genes that we suspect are linked on a single chromosome. What is the order of the genes and the distances between them? Steps: 1.Identify reciprocal gamete types. You know which is which 2.Consider each pair of genes separately, count recombinants. (count # recombinant gametes) 3.Identify the most distant genes as the ones with the greatest frequency of recombination. 4.Record distances between adjacent genes. 5.Account for double recombinant gametes (DR). (a) Identify DR that recombine only the MIDDLE marker. (b) Account for DR in total distance measure (c) Determine whether there is Interference Simple Scenario: Phase known Phase When we have a heterozygous individual for at least two genes, which chromosome is the allele on? ap3 + pi + ag + Clicker question: From a meiocyte with the following genotype: Which of the following has a cross-over event between ap3 and pi (gene order is unknown)?...
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This note was uploaded on 01/19/2012 for the course BIOLOGY 305 taught by Professor Kumar during the Fall '11 term at University of Michigan.

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09.27.11 - Lecture 7. Three point mapping, centromere...

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