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Lecture 7 Feb. 15, 2006

Lecture 7 Feb. 15, 2006. Three-point crosses. When genes are linked, parental combinations outnumber recombinant types. w + y + /Y. w + y /Y. w y /Y. w y + /Y. P. ♀ w y + / w y +. x. ♂ w + y /Y. F 1. ♀ w y + / w+ y. x. ♂ w y + /Y. F 2 males. 76. 53. 4484. 4413. w + y + /Y.

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Lecture 7 Feb. 15, 2006

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  1. Lecture 7Feb. 15, 2006 Three-point crosses

  2. When genes are linked, parental combinations outnumber recombinant types. w+ y+/Y w+ y/Y w y/Y w y+/Y P ♀w y+ / w y+ x ♂w+ y/Y F1 ♀w y+/w+y x ♂w y+/Y F2 males 76 53 4484 4413

  3. w+ y+/Y w+ y/Y w y/Y w y+/Y Calculation of recombination frequency F2 males 76 53 4484 4413 parental types:(4484 + 4413) / 9026 x 100 = 99% Recombinant types:(76 + 53) / 9026 x 100 = 1%

  4. w y/Y w+ y+/Y w+ y/Y w y+/Y 3472 3287 41 36 Designation of “parental” and “recombinant” relate to the history in a particular cross. Recombination frequency is independent of the original arrangement of the alleles in the parents P x ♂w y/Y ♀w+ y+ / w+ y+ F1 ♀w y/ w+ y+ x ♂w+ y+/Y F2 males recombination frequency: (41+36)/6838 = 1%

  5. Recombination frequency depends on the gene pair Look at recombination frequency of w (white-eyes) and m (miniature wings) which are both X-linked. P ♀ w+ m+ / w+ m+ x ♂ wm/ Y F1♀ w+ m+ / wm x ♂ w+ m+ / Y F2 males: 412 w+ m+ / Y 389 wm/ Y 206 w+ m / Y 185 wm+ / Y Total: 1192 Parental classes: (412+389)/1192 = 67.2% Recombinant classes: (206+185)/1192 = 32.8%

  6. Autosomal genes also exhibit linkage P ♀ bc+ / bc+ x ♂ b+ c/ b+ c F1 all bc+ / b+ c Test cross: ♀bc+ / b+ c x ♂ b c/ b c 2934 bc+/ b c 2768 b+ c/ b c 871 b c/ b c 846 b+ c+ / b c Total: 7419 Recombinant frequency: (871+846)/7419 x 100% = 23%

  7. Recombination frequencies of a pair of genes reflect the distance between them along a chromosome • Two point crosses are limited when genes are very close together or when double crossovers affect the recombination freq. • Three point crosses are more accurate in mapping genes

  8. A three-point testcross in corn Looking at the linked genes: • lz: lazy or prostrate growth • gl: glossy leaf • su: sugary endosperm A cross is done using multiply heterozygous parental genotype: LlGgSs x LlGgSs

  9. Progeny from 3-point testcross in corn Phenotype Genotype of testcross progeny of gamete Number • Normal Lz Gl Su 286 • Lazy lz Gl Su 33 • Glossy Lz gl Su 59 • Sugary Lz Gl su 4 • Lazy, glossy lz gl Su 2 • Lazy, sugary lz Gl su 44 • Glossy, sugary Lz gl su 40 • Lazy, glossy, sugary lz gl su 272 Total: 740

  10. In any genetic cross involving linked genes, no matter how complex, the two most frequent types of gametes with respect to any pair of genes are nonrecombinant: these provide the linkage phase (cis vs. trans) of the alleles of the genes in the multiply heterozygous parent. Progeny (corn gametes) from 3-point testcross: • Normal Lz Gl Su 286 • Lazy lz Gl Su 33 • Glossy Lz gl Su 59 • Sugary Lz Gl su 4 • Lazy, glossy lz gl Su 2 • Lazy, sugary lz Gl su 44 • Glossy, sugary Lz gl su 40 • Lazy, glossy, sugary lz gl su 272

  11. The double crossover gametes will be the least frequent types and can indicate the order of the three genes on the chromosome. A double crossover event will exchange the middle pair of alleles.  This testcross suggests that the Su gene is in the middle of the other two. Progeny (corn gametes) from 3-point testcross: • Normal Lz Gl Su 286 • Lazy lz Gl Su 33 • Glossy Lz gl Su 59 • Sugary Lz Gl su 4 • Lazy, glossy lz gl Su 2 • Lazy, sugary lz Gl su 44 • Glossy, sugary Lz gl su 40 • Lazy, glossy, sugary lz gl su 272

  12. Organizing the data • Parental types: • Normal Lz Su Gl 286 • Lazy, glossy, sugary lz su gl 272 • Single crossover between lz and su: • Glossy, sugary Lz su gl 40 • Lazy lz Su Gl 33 • Single crossover between su and gl: • Lazy, sugary lz su Gl 44 • Glossy Lz Su gl 59 • Double crossover types • (both lz x su and su x gl occurred) • Sugary Lz su Gl 4 • Lazy, glossy lz Su gl 2 10.7% 14.7%

  13. Building the map lz gl su 10.7 map units 14.7 map units

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