L9 Breakin' the Law! - Linkage and genetic mapping

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Unformatted text preview: Linkage and genetic mapping https://www.llnl.gov/str/May01/gifs/Stubbs2.jpg 1 Last Time Overview of gene interaction Epistasis - the masking effect Complementation - two genes, one phenotype Modifying genes - tweaking phenotypes Gene redundancy - the backup plan 2 Linkage and genetic mapping Breaking the Law of Independent Assortment What is linkage? Creating maps Frequency of recombination Determining the order of genes on chromosomes 3 Independent assortment http://fig.cox.miami.edu/~cmallery/150/mitosis/sf9x5b.jpg 4 Breaking the law...of independent assortment Linkage group Linked genes Genes located close together on a chromosome are generally inherited together 5 Crossing over The farther apart two genes are on a chromosome, the more likely crossing over events will occur between them http://www.answersingenesis.org/home/area/magazines/tj/images/v17n1_demographics.jpg 6 no recombination (no crossing over) recombination (crossing over) 7 Discovery of linked traits X dihybrid cross X Expectation? 8 Discovery of linked traits 9 Chi Square test X2 = (O1 - E1)2 + (O2 - E2)2 + (O3 - E3)2 + (O4 - E4)2 E2 E4 E4 E1 X2 = (296 - 240)2 +…. 2401 X2 = 219.2 10 Discovery that genes are physically located on the same chromosome P y = yellow body, y+ = grey body w = white eyes, w+ = red eyes m = mini-wings, m+ = long wings F1 Het Recessive 11 F2 results Het Recessive ? 12 Morgan’s assumptions All genes (body color, eye color, wing shape) are on X chromosome Crossing over on X chromosome results in physical exchange of DNA between homologous chromosomes The likelihood of crossing over depends on how far apart the genes are on the chromosome 13 14 distance is important! Crossing over between w+ (eyes) and m+ (wings) is fairly likely ...but crossing over between y+ (body) and w+(eyes) is unlikely 15 Double cross over Crossing over between w+ (eye) and m+ (wing) and also between y+ (body) and m+ (wing) is very, very unlikely 16 What does this mean? recap Het Recessive ? 17 Physical evidence of exchange of DNA between homologous chromosomes recombinant forms 18 Genetic maps an overview • Determining the order and position of genes on a chromosome • Maps are useful for a number of reasons 19 Frequency of recombination is correlated to the distance between genes. 20 s+ long bristles s short bristles + grey body e e ebony body Did recombination occur between these genes during meiosis of the heterozygous parent? 21 537 542 76 75 22 Determining map distance The Data: Parental phenotypes in F2: 537 + 542 Recombinant phenotypes in F2: 76 + 75 Map distance = # recomb. offspring X 100 Total # offspring 12.3 centiMorgans (cM) 23 Constructing a genetic map 1. Perform genetic cross (heterozygous x homozygous recessive for all genes of interest) 2. Analyze phenotypes 3. Calculate percentage of recombinant/total offspring 24 The cross 25 Putting it together y 0 1 w 1 29.7 v 3 m 23.9 r recomb freq inferred distance 30.7 33.7 57.6 Assume that map distances are more accurate between closely linked genes. 26 Trihybrid Crosses and Genetic Mapping 1. cross 2 true breeding strains that differ for 3 genes 2. perform testcross with F1 female het 3. collect F2 data 27 Creating a map Double cross over will be least frequent Combination tells us which gene is in the middle 28 Exercise b pr vg Choose genes closest together to position first (Keep in mind double crossover events can skew distances) 29 Summary Breaking the Law of Independent Assortment What is linkage? Creating maps Frequency of recombination Determining the order of genes on c’some 30 Ungraded problems Work through Chapter 4 S1, S2, S3A Try Chapter 4: C2, C7, C8, C15 Try Chapter 4: E11 31 Next Time Non-Mendelian inheritance Pt. 1 (Maternal and epigenetic inheritance) http://www.scienceinschool.org/repository/images/issue2epigenetics1_large.jpg 32 ...
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