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

2. 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

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

4. 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%

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 ♂ 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: ( )/1192 = 67.2%
Recombinant classes: ( )/1192 = 32.8%

6. 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: ( )/7419 x 100% = 23%

7. 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

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 su gl
10.7 map units
14.7 map units