Unformatted text preview: 204-325 1 Summary of the Last Lecture Quantitative genetics studies continuous phenotype, which are polygenic and each contributing genotype itself produces a range of phenotypes due to environment. Heritability is the proportion of the phenotypic variation in a population in a particular environment that is contributed by genetic variation. QTLs provide estimation of the number and relative importance of genes influencing a continuous phenotype.
2 Today's Topics 1. Linkage: Crossover between Chromatids and Recombination between Genes 2. Construct Genetic Maps Using Recombination Data 3 Topic 1 Linkage:
Crossover between Chromatids and Recombination between Genes
4 Gene Linkage: Deviation from Independent Assortment wm w+m+ w m+ w+m 5 Physical Crossing-over Produces Genetic Recombination
Crossover is the reciprocal exchange of homologous chromosomal segments Chiasma: Site of Crossover 6 Gene Recombination Correlates with Chromosomal Exchange 7 Genetic Recombination Results from Chromosomal Exchange Parental: Allelic combinations exist in either one of the parents Recombinant: Allelic combinations exist in neither one of the parents 8 Animation 1: Chromosomal Basis of Recombination 9 Recombination due to Independent Assortment of Unlinked Genes 10 Chi-Square Test Contradicts Independent Assortment b b b+ b vg vg + vg + vg + X X b b+ b b + vg vg vg vg
(Test Cross) b/b vg+/vg b+/b vg/vg b/b vg/vg b+/b vg+/vg 2= 11 (o-e)2 e P Value Table: Statistical Significance
P Values ... 1470 12 Animation 2: Chi-Square Test 13 Topic 2 Construct Genetic Maps Using Recombination Data 14 Recombination due to Independent Assortment of Unlinked Genes 15 Recombination due to Crossover of Linked Genes 16 Cis and Trans Dihybrids Produce Same Recombination Frequency (coupling) (repulsion) Recombination frequency is a quantitative estimation of genetic distance between Gene A and Gene B on chromosome 17 Relationship Between Crossing-over and Map Distance 18 The First Genetic Map (1913): Positioning Genes on Chromosome By Alfred Sturtevant, an undergraduate student at the time. 19 Gene Mapping Using Two Point Testcross count the number count the number 20 Recombination Frequency and Map Unit Recombination Frequency = Number of recombinants X 100 Numbers of Progenies Map Unit = Crossover Frequency Crossover Frequency is approximated by Recombination Frequency
21 Recombination Frequencies are 50 Percent or less
2 genes far apart on 1 chromosome have 50% recombination 2 genes on different chromosomes also have 50% recombination 22 Why Mendel did not See Gene Linkage? 7 1 5 3 4 6 2 23 Three-point Mapping ( p j r) Step 1 Determine gene order Step 2 Calculate recombination frequency in region 1 and region 2 respectively
24 Consequences of a Double Crossover: Exchange Middle Allele Pairs 25 Double Crossover is Least Frequent in 3-point Mapping
Class 7 & 8 must be the double crossover, j allele pair is exchanged in 7 & 8, therefore j must be in the middle of the three gene 26 Interpretation of the Results Based on Gene Order P J R (p to j) = (52 +46+4+2)/500 = 20.8% (j to r) = (22 +22+4+2)/500 = 10.0%
27 Procedure of Map Distance Calculation from 3 Point Cross 1. Find the lest frequent genotype pairs, which are results of double crossover (dco). 2. The exchange of only one pair of alleles can convert the parental genotypes to the dco genotypes (identified above). This pair is the middle of the three in the gene order. 3. Based on the gene order, single crossovers (sco) are either region 1 crossover or region 2 crossover 4.
Map Unit of Region 1 = all sco in region 1 + all dco
Total Progeny Map Unit of Region 2 = all sco in region 2 + all dco
Total Progeny 28 Animation 3: Three Point Mapping 29 Genetic Map of the p-j-r Region of the Chromosome p j r belong to a linkage group Genetic Map and Physical Map are related but not identical
30 Double Crossover Less than Expected Expected=(Single crossover between p and j ) X (Single crossover between j and r) = 0.208 X 0.100 = 0.0208 = 2.08 % Actual=(4+2)/500 =1.2%
31 Coincidence and Interference Coefficient of Coincidence = Observed double crossover frequency Expected double crossover frequency Interference = 1- Coefficient of Coincidence
32 Under Detection of Double Crossovers 33 Recombination Frequency could Underestimate Map Unit Recombination Frequency = Number of recombinants X 100 Numbers of Progenies Map Unit = Crossover Frequency
Recombination Frequency Crossover Frequency 34 Mapping Function: Map Distance and Recombination Frequency Assumption: Interference=0
35 Genetic Map of the Entire Drosophila Genome Genes mentioned in this course 36 Genetic Map of Tomato 37 Summary Genetic recombinants have allele combinations different from parental allele combinations. Recombination is the result either of independent assortment or of crossover. Crossover is the reciprocal exchanges between homologous chromosomes at the four chromatid stage of meiosis. One genetic map unit (mu or cM) is the interval in which 1% crossover occurs. Gene mapping crosses measure recombination frequencies, which are used to estimate the crossover frequencies of genes within 7 mu.
View Full Document
- Spring '08
- Genetics, Genetic linkage, Chromosomal crossover, recombination frequency, Map Unit