Lecture 7 Feb. 15, 2006

1
Lecture 7Feb. 15, 2006

• Three-point crosses

2
When genes are linked, parental combinations
outnumber recombinant types.
P
?w y / w y
?w y /Y
x
F1
x
?w y / w y
?w y /Y
F2 males
76
53
4484
4413
3
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
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 /Y
x
?w y / w y
F1
?w y / w y
x
?w y /Y
F2 males
recombination frequency (4136)/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 (412389)/1192 67.2
Recombinant classes (206185)/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 (871846)/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