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Chapter 7 Linkage, Crossing Over, and Chromosome Mapping in Eukaryotes

Chapter 7 Linkage, Crossing Over, and Chromosome Mapping in Eukaryotes. José A. Cardé, PhD Universidad Adventista de las Antillas Agosto 2013. Chapter Outline. Linkage, Recombination, and Crossing Over Chromosome Mapping. A Chromosome Map.

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Chapter 7 Linkage, Crossing Over, and Chromosome Mapping in Eukaryotes

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  1. Chapter 7Linkage, Crossing Over, and Chromosome Mapping in Eukaryotes José A. Cardé, PhD Universidad Adventista de las Antillas Agosto 2013

  2. Chapter Outline • Linkage, Recombination, and Crossing Over • Chromosome Mapping

  3. A Chromosome Map • Mapa genético- arreglo linear de un cromosoma con genes • Sturtevant - baso en el principio de que los genes q estan en un mismo cromosoma se deben heredar juntos • Estan fisicamente juntos PLT deben viajar juntos en meiosis

  4. Genes that are on the same chromosome travel through meiosis together; however, alleles of chromosomally linked genes can be recombined by crossing over. Linkage, Recombination, and Crossing Over

  5. Linkage and Recombination • Linkage • Recombination • Crossing over • Chiasma (chiasmata)

  6. Linked Genes Do NotAssort Independently -P – color de flores y longitud del polen F1 – rojas y largo – dominancia F2 – 24.3:1.1:1:7.1 X2>500 fenotipo parental: sobrerepresentado otros fenotipos: subrepresentado PLT : no hay sorteo independiente X2 = 7.8 vs 533.6

  7. Linked Genes Do NotAssort Independently • A que se debe la ausencia de sorteo independiente? • ambos genes estan en el mismo cromosoma = ligados • si los genes estan ligados se espera que los F1 solo tengan 2 gametos y no 4 • aparentemente cada cierto tiempo ocurre algo que produce combinaciones de donde salen dos gametos nuevos

  8. Testcross for linkage assestment • Cruce de Prueba • este análisis demuestra el genotipo de los gametos de los F1 doble heterocigotos • la progenie recombinante en F2 es solo 8% (muy bajo) del total, PLT los gene estan fuertemente ligados

  9. Linkage Phases:Coupling and Repulsion

  10. Hasta ahora: • La frecuencia de recombinación por las plantas heterocigotas de F1 se calcula dividiendo 80/1000 = 0.08 • genes mientras mas cercanos en el cromosoma están, menos probable q se separen PLT mas ligados están • Genes mas lejanos en el cromosoma, mas probable q se separen, menos intenso es el ligamiento

  11. Hasta ahora: • Para cualquiera 2 genes, la frecuencia de recombinación NUNCA excederá el 50% • este es el limite superior máximo y se obtiene cuando los genes están totalmente separados o sea en cromosomas diferentes!! • 50% de hecho es el % de recombinación que vemos cuando decimos que los gene se sortean independiente

  12. Crossing Over

  13. Multiple Crossovers

  14. Key Points • Linkage between genes is detected as a deviation from expectations based on Mendel’s Principle of Independent Assortment. • The frequency of recombination measures the intensity of linkage. • In the absence of linkage, this frequency is 50 percent; for very tight linkage, it is close to zero.

  15. Key Points • Recombination is caused by a physical exchange between paired homologous chromosomes early in prophase of the first meiotic division after chromosomes have duplicated. • At any one point along a chromosome, the process of exchange (crossing over) involves only two of the four chromatids in a meiotic tetrad. • Late in prophase I, crossovers become visible as chiasmata.

  16. Linked genes can be mapped on a chromosome by studying how often their alleles recombine. Chromosome Mapping

  17. Observable Outcomes of Crossing Over • Formation of chiasmata in late prophase. • Recombination between genes on opposites sides of the crossover point.

  18. Genetic Map Distances • The distance between two points on the genetic map of a chromosome is: the average number of crossovers between them.

  19. Cálculos de CO • La distancia entre dos puntos en un mapa genético de un cromosoma : el número promedio de CO entre ellos • 100 ovogonios • en algunos no hay CO = 70 • en otros hay 1 = 20 • en otros hay 2 = 8 • en otros hay 3 = 2 • al final calcular el número promedio de CO entre genes en los cromosomas analizados = 0.42

  20. Recombination Mapping with a Two-Point Testcross • vg+/b+ x vg/b • F1 vg+/b+ • vg+/b+ x vg/b (Test Cross) • 4 clases, 2 abundantes y 2 recombinantes • 180/ 1000 = 0.18 • recombinantes <<50% • (0)x0.82 + (1)x0.18 = 0.18

  21. Recombination Mapping • The Recombination Frequency between vg and b is 18% • This is equal to 18 map units, or 18 centiMorgans (cM) on the genetic map.

  22. Recombination Mapping with a Three-Point Testcross • sc+/ec+/cv+ x sc/ec/cv • F1 – sc+/ec+/cv+ • TestCross • 8 clases fenotipicas, 2 parentales y 6 recombinantes • cada clase recombinante representa algun tipo de CO • 1ro determinar el orden • 2do determinar que CO produce que recombinante

  23. Determining the Gene Order • There are 3 possible gene orders • sc - ec - cv • ec - sc - cv • ec - cv - sc • The two most common classes are the parentals. • Among the recombinant classes, the 2 rare classes represent the double crossovers. • The gene that is “switched” in the double crossover classes compared to the parental is the middle gene (in this case, ec).

  24. Calculation of Map Distances 0(1158+1455)/ 3248 + 1(163+130 + 192 + 148)/3248 + 2 (1+1+1+1)/3248 =0.196 x 100 = 19.6

  25. Interference and the Coefficient of Coincidence • Assuming independence, the expected frequency of double crossovers is 0.091  0.105 = 0.0095. • The observed frequency of double crossovers was 2/3248 = 0.0006. • A crossover in one region inhibited a crossover nearby.

  26. Coeficiente de Coincidencia • The coefficient of coincidence (c) is the ratio of observed double crossovers to expected double crossovers. c = 0.0006 / 0.0095 = 0.063 • Interference – un CO inhibe la ocurrencia de otro. • (I) = 1 - c I = 1 - 0.063 = 0.937

  27. Recombination Frequency and Genetic Map Distance La distancia en dos genes puede ser mas que lo que la frecuencia de recombinantes sugiera, PQ? PQ Esta se basa en el PROMEDIO de los CO Cuando es que la frecuencia de recombinacion no refleja la distancia real en un mapa cromosomico? Cuando estan demasiado lejos! Ej genes sc y f – separados por 66.8 cM (empírico basado en intermediarios) Segun el extpo y los resultados hay un 50% de recombinacion… o sea es como si estuvieran en cromosomas separados!!! Pero 50 cM no es igual a 66.8 cM que se saca estudiando las regiones entre ellos

  28. DCO y recombinantes Resultados de DCO entre dos puntos DCO entre 2 hebras = No Recom DCO entre 3 hebras = 50%/50% DCO entre 4 hebras = 100% de Recomb

  29. Recombination Frequency and Genetic Map Distance

  30. Key Points • The genetic maps of chromosomes are based on the average number of crossovers that occur during meiosis. • Genetic map distances are estimated by calculating the frequency of recombination between genes in experimental crosses.

  31. Key Points • Recombination frequencies less than 20 percent estimate map distance directly; however, recombination frequencies greater than 20 percent underestimate map distance because multiple crossover events do not always produce recombinant chromosomes. • An average of one chiasma during meiosis is equivalent to 50 centiMorgans of genetic map distance.

  32. Key Points • Recombination can bring favorable mutations together. • Chromosome rearrangements, especially inversions, can suppress recombination. • Recombination is under genetic control.

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