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Lecture 7
Feb. 15, 2006
Three-point crosses
When genes are linked, parental combinations
outnumber recombinant types.
P
♀w y+ / w y+
x
♂w+ y /Y
♀w y+ / w+ y
x
♂w y+ /Y
F1
F2
males
w y+ /Y
4484
w+ y /Y
4413
w+ y+ /Y
76
w y /Y
53
Calculation of recombination frequency
F2
males
w y+ /Y
4484
w+ y /Y
4413
parental types:
(4484 + 4413) / 9026 x 100 = 99%
Recombinant types:
(76 + 53) / 9026 x 100 = 1%
w+ y+ /Y
76
w y /Y
53
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
♀w+ y+ / w+ y+
x
♂w y /Y
♀w y / w+ y+
x
♂w+ y+ /Y
F1
F2
males
w y /Y
w+ y+ /Y
w+ y /Y
w y+ /Y
3472
3287
41
36
recombination
frequency:
(41+36)/6838
= 1%
Recombination frequency depends
on the gene pair
Look at recombination frequency of w (whiteeyes) and m (miniature wings) which are both
X-linked.
P ♀ w+ m+ / w+ m+
F1 ♀ w+ m+ / w m
F2 males:
412
389
206
185
Total: 1192
x
x
♂ wm/ Y
♂ w+ m+ / Y
w+ m+ / Y Parental classes: (412+389)/1192 = 67.2%
wm/ Y
w+ m / Y
Recombinant classes: (206+185)/1192 = 32.8%
+
wm / Y
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%
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
of testcross progeny
 Normal
 Lazy
 Glossy
 Sugary
 Lazy, glossy
 Lazy, sugary
 Glossy, sugary
 Lazy, glossy, sugary
Genotype
of gamete
Number
Lz Gl Su
286
lz Gl Su
33
Lz gl Su
59
Lz Gl su
4
lz gl Su
2
lz Gl su
44
Lz gl su
40
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
•Lazy, glossy, sugary lz su gl
Single crossover between lz and su:
•Glossy, sugary
Lz su gl
•Lazy
lz Su Gl
Single crossover between su and gl:
•Lazy, sugary
lz su Gl
•Glossy
Lz Su gl
Double crossover types
(both lz x su and su x gl occurred)
•Sugary
Lz su Gl
•Lazy, glossy
lz Su gl
286
272
40
33
44
59
4
2
10.7%
14.7%
Building the map
lz
gl
su
10.7 map units
14.7 map units