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Fig. 4-1
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  1. Chapter 4 overview Fig. 4-1

  2. Genetic recombination: mixing of genes during • gametogenesis that produces gametes with • combinations of genes that are different from • the combinations received from parents. • Independent assortment of homologous • chromosomes (Anaphase I). Genes on non- • homologous chromosomes (unlinked genes) • assort independently.

  3. Fig. 4-6

  4. Using a testcross to distinguish gamete genotypes Fig. 4-7

  5. 50% = independent assortment (genes are not linked) Fig. 4-8

  6. Genetic recombination: mixing of genes during • gametogenesis that produces gametes with • combinations of genes that are different from • the combinations received from parents. • Independent assortment of homologous • chromosomes (Anaphase I). Genes on non- • homologous chromosomes (unlinked genes) • assort independently. • Crossing over (recombination among linked • genes)

  7. cis linked: both dominant alleles on the same homolog trans linked: dominant alleles on different homologs Fig. 4-2

  8. Fig. 4-3

  9. Crossing over • Physical exchanges among non-sister chromatids; • visualized cytologically as chiasmata • Typically, several crossing over events occur within • each tetrad in each meiosis (chiasmata physically • hold homologous chromosome together and assure • proper segregation at Anaphase I) p. 115

  10. Crossing over occurs at the four-strand stage (pre-meiotic G2 or very early prophase I) Fig. 4-4

  11. Crossing over can involve 2, 3, or 4 chromatids in a single meiosis Fig. 4-5

  12. Crossing over • Physical exchanges among non-sister chromatids; • visualized cytologically as chiasmata • Typically, several crossing over events occur within • each tetrad in each meiosis (chiasmata physically • hold homologous chromosome together and assure • proper segregation at Anaphase I) • The sites at which crossing over occur are random • The likelihood that a crossover occurs between any • two particular sites (genes) is a function of the • physical distance between those two sites

  13. Crossing over usually affects a minority of chromatids in a collection of meioses – recombinants are typically a minority of products Fig. 4-9

  14. <50% = linked genes Fig. 4-10

  15. A.H. Sturtevant (1911-3): frequency of crossing over between two genes is a function of their distance apart on the chromosome; created the first genetic map number of recombinants Recombination frequency = total number of progeny One map unit = one centimorgan = 1% recombinants

  16. Rationales: • Crossover events are random • Greater separation, greater likelihood that crossover will occur • Map distance should be sum of smaller intervals • Construct entire chromosome maps by mapping intervals • Linear map correlates with linear chromosome Fig. 4-11

  17. Markers used in trihybrid testcross in Drosophila v = vermilion eyes (red eyes; v+are red-brown) cv = crossveinless (cv+ wings have crossveins) ct = cut wing (ct+ wings have regular margins)

  18. Data from three-point testcross v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester) Progeny phenotypes v cv+ ct+ 580 v+ cv ct 592 v cv ct+ 45 v+ cv+ ct 40 v cv ct 89 v+ cv+ ct+ 94 v cv+ ct 3 v+ cv ct+ 5 1448

  19. Steps in solving three-point testcross problem • Anticipate and identify eight types of products (23) • Identify pairs of reciprocal products

  20. Data from three-point testcross v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester) Progeny phenotypes v cv+ ct+ 580 v+ cv ct 592 v cv ct+ 45 v+ cv+ ct 40 v cv ct 89 v+ cv+ ct+ 94 v cv+ ct 3 v+ cv ct+ 5 1448

  21. Steps in solving three-point testcross problem • Anticipate and identify eight types of products (23) • Identify pairs of reciprocal products • Identify parental types as the most frequent pair of • products • Identify double crossover products as least frequent • pair of products

  22. Data from three-point testcross v+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester) Progeny phenotypes v cv+ ct+ 580 v+ cv ct 592 v cv ct+ 45 v+ cv+ ct 40 v cv ct 89 v+ cv+ ct+ 94 v cv+ ct 3 v+ cv ct+ 5 1448 Parental types - nco sco sco dco

  23. Steps in solving three-point testcross problem • Anticipate and identify eight types of products (23) • Identify pairs of reciprocal products • Identify parental types as the most frequent pair of • products • Identify double crossover products as least frequent • pair of products • Compare the parental and double crossover products • to deduce the order of the three gene loci

  24. Fig. 4-12 In dco products, the central marker is displaced relative to the parental types

  25. Fig. 4-13

  26. Steps in solving three-point testcross problem • Anticipate and identify eight types of products (23) • Identify pairs of reciprocal products • Identify parental types as the most frequent pair of • products • Identify double crossover products as least frequent • pair of products • Compare the parental and double crossover products • to deduce the order of the three gene loci • Compute map distances by breaking down the • results for each interval

  27. Fig. 4-12 85 + 8 1448 (0.064) 183 + 8 1448 (0.132) RF =

  28. Fig. 4-12 85 + 8 1448 (0.064) 183 + 8 1448 (0.132) RF = 13.2 m.u. 6.4 m.u. v ct cv

  29. Interference: crossing over in one region interferes with simultaneous crossing over in adjacent regions Expected frequency of dco = product of frequency crossovers in two regions 0.132 X 0.064 = 0.0084 0.084 X 1448 = 12 expected (if two sco are independent events)

  30. Interference: crossing over in one region interferes with simultaneous crossing over in adjacent regions Expected frequency of dco = product of frequency crossovers in two regions 0.132 X 0.064 = 0.0084 0.084 X 1448 = 12 expected (if two sco are independent events) Coefficient of coincidence = observed dco / expected dco 8 / 12 = 0.667 Interference = 1 – coefficient of coincidence 1 – 0.667 = 0.333

  31. Fig. 4-14 Tomato karyotype (n=12)

  32. Tomato linkage map (1952) Fig. 4-14

  33. Typical phenotypic ratios for a variety of crosses (complete allele dominance) p. 136