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Download Lecture#17 Page 1 BIOLOGY 207 – Dr McDermid Lecture#17
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BIOLOGY 207 – Dr McDermid Lecture#17 Linkage Readings: Griffiths et al, 7th Edition: Ch. 5 pp 141-149 Problems: Ch 5: Tier 1: #1, 2, 5, 6, 9, 35, 44 Tier 2: #3, 4, 7, 8, 36, 39, 40, 46 (note: 7b answer is wrong) Concepts: Gene loci on the same chromosome : 1. Gene loci on the same chromosome may show linkage, not independent assortment. 2. Most linkage between gene loci is not complete because crossing over between loci can occur during meiosis. 3. The extent of linkage between gene loci is expressed as the frequency of recombinant type progeny (vs. parental type progeny). Independent Assortment - two gene loci, on separate chromosomes - alleles at each locus segregate independently But if the two gene loci are on the same chromosome then the alleles at the two different gene loci should segregate together (for the most part) in meiosis. This would result in an individual having the tendency for particular allele combinations to be passed on to its offspring. Example: AABB x AaBb Then test cross the F1 aabb F1 P1 A word about notation A and B -> two loci on separate chromosomes AaBb is A/a B/b A and B on one chromosome AB/ab represents alleles A and B on one homolog and a and b on the other homolog. Coupling conformation Repulsion conformation Test cross: AaBb x aabb gametes Genotype Phenotype ab gametes AB AaBb AB Ab Aabb Ab aB aaBb aB ab aabb ab If A + B were assorting independently, But if A, B were on the same chromosome, Gametes: AB from AaBb ab from aabb Ab from Aabb aB from aaBb There are two types of gametes: parentals - which are the same organization as in parent chromosomes Lecture#17 Page 1 recombinants - a cross over occurs between the A and B loci to give recombinant gametes Ab and aB. Note: 1) the two parental combinations should occur equally likely (1:1 ratio) 2) the two recombinant combinations should also occur with equal frequency 1) When the two loci are very close together Most meiotic products : Ratios of gametes in test cross: Gametes Genotyp # progeny AB AaBb 98 ab aabb 98 Ab Aabb 2 aB aBb 2 # recombinants = total # progeny = Recombinant Frequency (RF) = (# recombinants/# total) 2) When the two loci are further apart on the same chromosome AC/AC X ac/ac AC/ac X ac/ac Ratios of gametes: Gametes Genotyp AC AaCc ac aacc Ac Aacc aC aCc Recombinant Frequency = # recombinants / total # progeny Lecture#17 Totals Parentals 196 Recombinants 4 Total= 200 P1 P2 # progeny 72 68 31 29 Totals Parentals 140 Recombinants 60 Total= 200 Page 2 Summarize: A - B loci A --------- C loci Allows us to position gene loci relative to each other along the chromosome A <---------------------------> C A <--->B Construct a genetic map or linkage map This was first done by Alfred Sturtevant in 1913 A genetic map is a representation of the physical relationships among linked genes along a chromosome. Any locus can be mapped relative to another on a chromosome. Map units 1% recombination is about 1 map unit. So A --> B A --> C Approximate because the frequency of the recombinants only approximates the frequency of crossing over. Example If two loci are unlinked, then the ratio of parentals: recombinants is the maximum recombinants is Two loci that are >50 map units apart -A-B---------C--------------D---•-----------E-------------F-Summary Unlinked: shows independent assortment Non-syntenic --•-------A--------!!!!!!!!!!!!!!!!!!!!--•------B------•-------a--------!!!!!!!!!!!!!!!!!!!!--•------b----Or syntenic -----A---------------------•----------------B---------a---------------------•----------------b---->50 map units Linked: doesn't show independent assortment Closely linked: -------•--------------A-B-----------•--------------a-b----- e.g. 2% Partially linked: -------•---------A------B-----------•---------a------b----- e.g. 30% "Synteny" - linked genes are syntenic - syntenic genes are not necessarily linked ______________________________________________________________________________________________ Lecture notes: Copyright © 2002 Heather McDermid and the Department of Biological Sciences, University of Alberta Images are Copyright©2000 by W.H. Freeman & Co. in Griffiths et al, Introduction to Genetic Analysis Lecture#17 Page 3