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ch06notes.ppt
1.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Chapter 6 Genetic Recombination in Eukaryotes Linkage and genetic diversity
2.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Overview • In meiosis, recombinant products with new combinations of parental alleles are generated by: – independent assortment (segregation) of alleles on nonhomologous chromosomes. – crossing-over in premeiotic S between nonsister homologs. • In dihybrid meiosis, 50% recombinants indicates either that genes are on different chromosomes or that they are far apart on the same chromosome. • Recombination frequencies can be used to map gene loci to relative positions; such maps are linear. • Crossing-over involves formation of DNA heteroduplex.
3.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Recombination (1) • A fundamental consequence of meiosis – independent assortment (independent segregation) – crossing-over between homologous chromatids • Yields haploid products with genotypes different from both of the haploid genotypes that originally formed the diploid meiocyte N N 2N N N N N different genotypes parentals recombinants meiosis
4.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Recombination (2) • Bringing together of two or more pairs of alleles into new combinations A/a B/b a/a b/b A/A B/B A B a b A B a b A b a B parental (P) genotypes recombinant (R) genotypes parental genotypes meiosis meiosis meiosis
5.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Independent assortment (1) • Also known as independent segregation • Consequence of independent alignment of chromosomes in meiotic bivalents A/A ; B/B a/a ; b/b A/a ; B/b ¼ A ; B P ¼ A ; b R ¼ a ; B R ¼ a ; b P OR Alternate bivalants A B b B a a A b
6.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Independent assortment (2) • For genes on different (nonhomologous) pairs of chromosomes, recombinant frequency is always 50% A/A ; B/B a/a ; b/b A/a ; B/b ¼ A ; B P ¼ A ; b R ¼ a ; B R ¼ a ; b P A/A ; b/b a/a ; B/B A/a ; B/b ¼ A ; B R ¼ A ; b P ¼ a ; B P ¼ a ; b R 50% recombinants
7.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Dihybrid testcross (1) • Determines genotype of dihybrid by crossing to homozygous recessive tester A/A ; b/b a/a ; B/B A/a ; B/b a/a ; b/b testcross Parental F1 F1 gametes tester gametes a ; b progeny proportions progeny phenotypes ¼ A ; B A/a ; B/b ¼ A B ¼ A ; b A/a ; b/b ¼ A b ¼ a ; B a/a ; B/b ¼ a B ¼ a ; b a/a ; b/b ¼ a b 1:1:1:1 ratio
8.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Dihybrid testcross (2) • Best way to study recombination is in a dihybrid testcross – only dihybrid produces recombinant genotypes – all homozygous recessive tester gametes alike • Typical 1:1:1:1 ratio a result of independent assortment in dihybrid • Each genotype in progeny has unique phenotype • Observed by Mendel in testcrosses with two pairs of traits
9.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Dihybrid selfing • Cross between two A/a ; B/b dihybrids – recombination occurs in both members of cross – recombination frequency is 50% A ; B A ; b a ; B a ; b A ; B A/A ; B/B A/A ; B/b A/a ; B/B A/a ; B/b A ; b A/A ; B/b A/A ; b/b A/a ; B/b A/a ; b/b a ; B A/a ; B/B A/a ; B/b a/a ; B/B a/a ; B/b a ; b A/a ; B/b A/a ; b/b a/a ; B/b a/a ; b/b Ratio: 9 A/– ; B/– 3 A/– ; b/b 3 a/a ; B/– 1 a/a ; b/b
10.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Product rule • Multiply probabilities of independent occurrences to obtain probability of joint occurrence • E.g. branched tree or grid methods • For mating A/a ; B/b A/a ; B/b – Segregation at A, gives ¾ A/– and ¼ a/a in progeny – Segregation at B, gives ¾ B/– and ¼ b/b in progeny ¾ A/– ¼ a/a ¾ B/– 9/16 A/– ; B/– 3/16 a/a ; B/– ¼ b/b 3/16 A/– ; b/b 1/16 a/a ; b/b
11.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Independent assortment: multiple loci • Calculations can be made for any gene combination using predicted outcomes at single loci and the product rule P1 A/a ; B/b ; C/c ; D/d P2 a/a ; B/b ; C/c ; D/D # gametes P1 2 x 2 x 2 x 2 = 16 # gametes P2 1 x 2 x 2 x 1 = 4 # genotypes in F1 2 x 3 x 3 x 2 = 36 # phenotypes in F1 2 x 2 x 2 x 1 = 8 Frequency of A/– ; B/– ; C/– ; D/– ½ x ¾ x ¾ x 1 = 9/32
12.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Deducing genotypes from ratios • Genetic analysis works in two directions – predict genotypes in offspring – determine genotypes of parents in cross • Specific expectations, e.g., 1:1:1:1 and 9:3:3:1 can be used to deduce genotypes • Testcross example: Phenotype # observed A/– ; B/– 310 A/– ; b/b 295 a/a ; B/– 305 a/a ; b/b 290 The observed results are close to 1:1:1:1, allowing the deduction that the tested genotype was a dihybrid.
13.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Crossing-over (CO) • Breakage and rejoining of homologous DNA double helices • Occurs only between nonsister chromatids at the same precise place • Visible in diplotene as chiasmata • Occurs between linked loci on same chromosome – cis: recessive alleles on same homolog (AB/ab) – trans: recessive alleles on different homologs (Ab/aB)
14.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Cis – trans crossing-over • Drawing shows only chromatids engaged in crossing-over • Effect is to switch between cis and trans A b a B a b A B cis trans A B a b a B A b meiotic crossing-over AB/ab aB/Ab Ab/aB AB/ab
15.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Cis dihybrid crossing-over • Parental (P) and recombinant (R) classes each have both alleles at each locus (reciprocal) • Each crossover meiosis yields two P chromosomes and two R chromosomes • Because CO does not occur in each meiocyte, frequency of recombinants (R) must be <50% A b a B A b a B P R R P A b a B
16.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Crossing-over • No loss of genetic material, just formation of new chromatids • Parental chromatids are noncrossover products • Recombinant chromatids are always products of crossing-over • All four genes (A, B, a and b) are present between both parental chromatids and between both recombinant chromatids
17.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Consequences of crossing-over • Frequency of recombinant gametes is 0- 50%, depending on frequency of meiocytes with crossing-over • Results in deviation from 1:1:1:1 in testcrosses – parental combination is most frequent – recombinant combination is rarest • Allows drawing of linkage maps based on recombination frequencies (RF)
18.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Recombination frequency (RF) • Experimentally determined from frequency of recombinant phenotypes in testcrosses • Roughly proportional to physical length of DNA between loci • Greater physical distance between two loci, greater chance of recombination by crossing-over • 1% recombinants = 1 map unit (m.u.) • 1 m.u. = 1 centiMorgan (cM)
19.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Linkage maps • RF is (60+50)/400=27.5%, clearly less than 50% • Map is given by: # observed 140 50 60 150 A B 27.5 m.u.
20.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Mapping • RF analysis determines relative gene order • RF between same two loci may be different in different strains or sexes • RF values are roughly additive up to 50% – multiple crossovers essentially uncouple loci, mimicking independent assortment • Maps based on RF can be combined with molecular and cytological analyses to provide more precise locations of genes
21.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Trihybrid testcross • Sometimes called three-point testcross • Determines gene order as well as relative gene distances • 8 categories of offspring – for linked genes, significant departure from 1:1:1:1:1:1:1:1 • Works best with large numbers of offspring, as in fungi, Drosophila
22.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Analysis of trihybrid testcross data • Identify pairs of parental and recombinant offspring – parental (noncrossover); most abundant – double crossovers; least abundant – single crossovers; intermediate abundance • identify on the basis of reciprocal combinations of alleles • Determine gene order by inspection (the parental gene order yields double crossovers by switching middle genes) • Calculate RF for single crossovers, adding double crossovers each time • Draw map
23.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Interference • Crossing-over in one region of chromosome sometimes influences crossing-over in an adjacent region • Interference = 1 – (coefficient of coincidence) • Usually, I varies from 0 to 1, but sometimes it is negative, meaning double crossing-over is enhanced ts recombinan double expected # ts recombinan double observed # c.o.c.
24.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Genetic maps • Useful in understanding and experimenting with the genome of organisms • Available for many organisms in the literature and at Web sites • Maps based on RF are supplemented with maps based on molecular markers, segments of chromosomes with different nucleotide sequences
25.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Chi-square test • Statistical analysis of goodness of fit between observed data and expected outcome (null hypothesis) • Calculates the probability of chance deviations from expectation if hypothesis is true • 5% cutoff for rejecting hypothesis – may therefore reject true hypothesis – statistical tests never provide certainty, merely probability
26.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Chi-square application to linkage • Null hypothesis for linkage analysis – based on independent assortment, i.e., no linkage – no precise prediction for linked genes in absence of map • for all classes • Calculated from actual observed (O) and expected (E) numbers, not percentages E E O 2 2 ) (
27.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Mechanism of meiotic crossing-over • Exact mechanism with no gain or loss of genetic material • Current model: heteroduplex DNA – hybrid DNA molecule of single strand from each of two nonsister chromatids – heteroduplex resolved by DNA repair mechanisms • May result in aberrant ratios in systems that allow their detection
28.
Chapter 6: Eukaryote
recombination © 2002 by W. H. Freeman and Company Recombination within a gene • Recombination between alleles at a single locus • In diploid heterozygous for mutant alleles of the same gene, recombination can generate wild-type and double mutant alleles • Rare event, 10-3 to 10-6, but in systems with large number of offspring, recombination can be used to map mutations within a gene a1/a2 a+ and a1,2
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