Lecture 7 Feb. 15, 2006 Three-point crosses

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

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

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 3472 w y /Y 3287 w+ y+ /Y w+ y /Y 41 36 w y+ /Y recombination frequency: (41+36)/6838 = 1%

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 ♂ w m / Y F1 ♀ w+ m+ / w m x ♂ w+ m+ / Y F2 males: 412 w+ m+ / Y 389 w m / Y 206 w+ m / Y 185 w m+ / Y Total: 1192 Parental classes: (412+389)/1192 = 67.2% Recombinant classes: (206+185)/1192 = 32.8%

Autosomal genes also exhibit linkage P ♀ b c+ / b c+ x ♂ b+ c / b+ c F1 all b c+ / b+ c Test cross: ♀b c+ / b+ c x ♂ b c / b c 2934 b c+/ 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%

Three point crosses are more accurate in mapping genes 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

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

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

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

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

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%

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