linkage
TRANSCRIPT
Linkage
A testcross from a dihybrid cross can be illustrated as follows:
P AABB x aabb A B a b
A B a b
F1 AaBb A B
a b
Test-cross AaBb x aabb
A B a b
a b a b
X
X
A B a b Test Crossa b a b
A B
a b a b Gametes
A b
a B
Test-cross progeny in 1 : 1 : 1 : 1 ratio
A B
a b
a b 50% of progeny have parental phenotype
a b
A b
a b 50% of progeny have recombinant phenotype
a B
a b
X
X
A B a b A B
a b a b a b
A B A b
X a b X
a b a B
Test-cross progeny not in 1 : 1 : 1 : 1 ratio
A B A b
a b a b
a b a B
a b a b
› 50% with parental phenotype ‹ 50% with recombinant phenotype
Cross-over
X X
reco
mbi
nant
reco
mbi
nant
Two-Factor Cross
re red-eye re+ wild-type
bz bronze-body bz+ wild-type
P rere/bzbz x re+re+/bz+bz+
F1 rere+/bzbz+ re bz re bz
Test-cross rere+/bzbz+ x rere/bzbz re+ bz+ re+ bz+
rebz (red-eye, bronze-body) 350 re bz
re+bz+ (wild-type) 350 re+ bz+
rebz+ (red-eye) 150 re bz+
re+bz (bronze-body) 150 re+ bz
1000
Recombination percentage = (150 + 150)/1000 x 100% = 30%
Map distance between re and bz = 30 map units
Cross A Cross B
wild type purple/vestigial
P pr+pr+vg+vg+ x prprvgvg
wild type purple/vestigial
TC pr+prvg+vg x prprvgvg
Wild type 162
Purple eye/vestigial 158
Normal eye/vestigial 39
Purple eye/normal wing 41
wild type purple/ebony
P pr+pr+e+e+ x prpree
wild type purple/ebony
TC pr+pre+e x prpree
Wild type 103
Purple eye/ebony 98
Normal eye/ebony 100
Purple eye/normal body 99
Parental phenotype
Recombinant phenotype
Observed 320 80
1. Null hypothesis: Parental phenotype and recombinant phenotype is in 1 : 1 ratio.
2.
3. 2 = (320-200)2/200 + (80-200)2/200
= 72 + 72 = 144
4. From the 2 table, 2 at 5% level of confidence with 1 degree of freedom is 3.84.
5. The null hypothesis is rejected, the parental and recombinant phenotype does not fit the 1 : 1 ratio.
Parental Recombinant
Observed 320 80
Expected 200 200
P b+ vg+ bw+ b vg bw
b+ vg+ bw+ b vg bw
Test- cross b+ vg+ bw+ b vg bw
b vg bw b vg bw
Progeny b+ vg+ bw+ 225 parental phenotype
b vg bw 220 (no cross-over)
b+ vg+ bw 89
b vg bw+ 86
b+ vg bw 80
b vg+ bw+ 85
b+ vg bw+ 7
b vg+ bw 8
800
Distance between vg - bw = X 100 = 23.75
b black body
b+ gray body
vg vestigial
vg+ normal wing
bw brown-eye
bw+ red-eye
vg – bw cross-over
b – vg cross-over
}
}
}
} double cross-over
(89+86+7+8)
800
X
X
Progeny b+ vg+ bw+ 225 parental phenotype
b vg bw 220 (no cross-over)
b+ vg+ bw 89
b vg bw+ 86
b+ vg bw 80
b vg+ bw+ 85
b+ vg bw+ 7
b vg+ bw 8
800
Distance between vg - bw = x 100 = 23.75
Distance between b – vg = x 100 = 22.50
Distance between b – bw = x 100 = 46.25
b black body
b+ gray body
vg vestigial
vg+ normal wing
bw brown-eye
bw+ red-eye
vg – bw cross-over
b – vg cross-over}
}
}
}
double cross-over
(89+86+7+8)
800
(80+85+7+8)
800
89+86+80+85+2(7+8)
800
b vg bw
22.50 23.75
Genetic map or Linkage map
Chromosome interference(Chiasma interference)
Observed double cross-over = = 0.0187
Expected double cross-over = 0.2250 x 0.2375 = 0.0534
Coefficient of interference = 1 – coefficient of coincidence
Coefficient of coincidence = =Observed double cross-over 0.0185
Expected double cross-over 0.0534
(7+8)
800
P b+ vg+ bw+ b vg bw
b+ vg+ bw+ b vg bw
Test- cross b+ vg+ bw+ b vg bw
b vg bw b vg bw
Progeny b+ vg+ bw+ 225 parental phenotype
b vg bw 220 (no cross-over)
b+ vg+ bw 89
b vg bw+ 86
b+ vg bw 80
b vg+ bw+ 85
b+ vg bw+ 7
b vg+ bw 8
800
b black body
b+ gray body
vg vestigial
vg+ normal wing
bw brown-eye
bw+ red-eye
vg – bw cross-over
b – vg cross-over
}
}
}
} double cross-over
X
X
P vg b bw+ vg+ b+ bw
vg b bw+ vg+ b+ bw
Test- cross vg b bw+ vg b bw
vg+ b+ bw vg b bw
Progeny vg b bw+ 225
vg+ b+ bw 220
vg b bw 89
vg+ b+ bw+ 86
vg+ b bw 80
vg b+ bw+ 85
vg b+ bw 7
vg+ b bw+ 8
800
}
}
}
} double cross-over
no cross-over
b black body
b+ gray body
vg vestigial
vg+ normal wing
bw brown-eye
bw+ red-eye
X
X
}
}
}
} double cross-over
no cross-overPhenotype No.
1. vg+ b+ 306
2. vg b 314
3. vg+ b 88
4. vg b+ 92
X
X
P vg b bw+ vg+ b+ bw
vg b bw+ vg+ b+ bw
Test- cross vg b bw+ vg b bw
vg+ b+ bw vg b bw
vg b bw+ 225
1 vg+ b+ bw 220
vg b bw 89
2 vg+ b+ bw+ 86
vg+ b bw 80
3 vg b+ bw+ 85
vg b+ bw 7
4 vg+ b bw+ 8
800
P vg b bw+ vg+ b+ bw
vg b bw+ vg+ b+ bw
Test- cross vg b bw+ vg b bw
vg+ b+ bw vg b bw
A B C
Progeny vg b bw+ 225 a b c
vg+ b+ bw 220
vg b bw 89 A B C
vg+ b+ bw+ 86 a b c
vg+ b bw 80
vg b+ bw+ 85 A b C
vg b+ bw 7 a B c
vg+ b bw+ 8
800
}
}
}
} double cross-over
no cross-over
X
X
P b vg bw+ b+ vg+ bw
b vg bw+ b+ vg+ bw
Test- cross b vg bw+ b vg bw
b+ vg+ bw b vg bw
Progeny b vg bw+ 225 parental phenotype
b+ vg+ bw 220 (no cross-over)
b vg bw 89
b+ vg+ bw+ 86
b vg+ bw 80
b+ vg bw+ 85
b+ vg bw 7
b vg+ bw+ 8
800
b black body
b+ gray body
vg vestigial
vg+ normal wing
bw brown-eye
bw+ red-eye
vg – bw cross-over
b – vg cross-over
}
}
}
} double cross-over
X
X
Progeny b vg bw+ 225 parental phenotype
b+ vg+ bw 220 (no cross-over)
b vg bw 89
b+ vg+ bw+ 86
b vg+ bw 80
b+ vg bw+ 85
b+ vg bw 7
b vg+ bw+ 8
800
Distance between vg - bw = x 100 = 23.75
Distance between b – vg = x 100 = 22.50
Distance between b – bw = x 100 = 46.25
b black body
b+ gray body
vg vestigial
vg+ normal wing
bw brown-eye
bw+ red-eye
vg – bw cross-over
b – vg cross-over}
}
}
}
double cross-over
(89+86+7+8)
800
(80+85+7+8)
800
89+86+80+85+2(7+8)
800