Linkage and Three-Point Mapping
Linkage and Three-Point Mapping
Three-point cross
o P1 – aa bb cc x ++ ++ ++
o F1 - +a +b +c
Triple heterozygous wild-types
o F2 – backcross to the mutant aa bb cc stock
Expect an equal number of the 8 possible combinations of phenotypes
1:1:1:1:1:1:1:1 ratio
Scute (sc), echinus (ec), crossveinless (cv)
o sc-ec-cv three-point cross – mapping position of loci involved between them
Expected result if no linkage
P1 – scute echinus crossveinless x wildtype
o Scsc ecec cvcv x ++ ++ ++
F1 – all wild type triple heterozygotes
o sc+ ec+ cv+
F2 – back cross with scute echinus crossveinless tester stock
o Expect if free recombination
Equal numbers of all possible combinations – 2 3
o Observe
Excess of parental types
scute echinus crossveinless
normal normal normal
Shortage of recombinant types
Observe
Scute to echinus
o Parentals
sc ec = A plus E = 417 plus 44
+ + = B plus F = 430 plus 37
Total 928
o Recombinants
sc + = C = 25
+ ec = D = 29
Total 54
o Total flies
982
o Map distance between scute to echinus = 54/982 = 5.5 map units
Echinus to crossveinless
o Parentals
ec cv = A plus D = 417 plus 29
+ + = B + C = 430 plus 25
Total 901
o Recombinants
, Linkage and Three-Point Mapping
ec + = E = 44
+ cv = F = 27
Total 81
o Total flies
982
o Map distance between echinus and crossveinless = 81/982 = 8.2 map units
Scute to crossveinless
o Parentals
sc cv = A = 417
+ + = B = 430
Total 847
o Recombinants
sc + = C plus E = 25 plus 44
+ cv = D plus F = 37 plus 37
Total 135
o Total flies
982
o Map distance between echinus and crossveinless = 135/982 = 13.7 map
units
Scute-echinus-crossveinless map
Crossveinless (cv), cut (ct), vermilion (ve)
o cv, ct, ve three point cross – mapping position of loci involved between them
Expected result if no linkage
P1 – crossveinless cut vermilion x wild type
o cvcv ctct veve x ++ ++ ++
F1 – all wild type triple heterozygous cv+ ct+ ve+
F2 – back cross with crossveinless cut vermilion tester stock
o Expect if free recombination
Equal numbers of all 8 possible combinations
o Observe
Parentals
o cv ct ve
o +++
Single crossovers – one recombination event
o cv + +
Linkage and Three-Point Mapping
Three-point cross
o P1 – aa bb cc x ++ ++ ++
o F1 - +a +b +c
Triple heterozygous wild-types
o F2 – backcross to the mutant aa bb cc stock
Expect an equal number of the 8 possible combinations of phenotypes
1:1:1:1:1:1:1:1 ratio
Scute (sc), echinus (ec), crossveinless (cv)
o sc-ec-cv three-point cross – mapping position of loci involved between them
Expected result if no linkage
P1 – scute echinus crossveinless x wildtype
o Scsc ecec cvcv x ++ ++ ++
F1 – all wild type triple heterozygotes
o sc+ ec+ cv+
F2 – back cross with scute echinus crossveinless tester stock
o Expect if free recombination
Equal numbers of all possible combinations – 2 3
o Observe
Excess of parental types
scute echinus crossveinless
normal normal normal
Shortage of recombinant types
Observe
Scute to echinus
o Parentals
sc ec = A plus E = 417 plus 44
+ + = B plus F = 430 plus 37
Total 928
o Recombinants
sc + = C = 25
+ ec = D = 29
Total 54
o Total flies
982
o Map distance between scute to echinus = 54/982 = 5.5 map units
Echinus to crossveinless
o Parentals
ec cv = A plus D = 417 plus 29
+ + = B + C = 430 plus 25
Total 901
o Recombinants
, Linkage and Three-Point Mapping
ec + = E = 44
+ cv = F = 27
Total 81
o Total flies
982
o Map distance between echinus and crossveinless = 81/982 = 8.2 map units
Scute to crossveinless
o Parentals
sc cv = A = 417
+ + = B = 430
Total 847
o Recombinants
sc + = C plus E = 25 plus 44
+ cv = D plus F = 37 plus 37
Total 135
o Total flies
982
o Map distance between echinus and crossveinless = 135/982 = 13.7 map
units
Scute-echinus-crossveinless map
Crossveinless (cv), cut (ct), vermilion (ve)
o cv, ct, ve three point cross – mapping position of loci involved between them
Expected result if no linkage
P1 – crossveinless cut vermilion x wild type
o cvcv ctct veve x ++ ++ ++
F1 – all wild type triple heterozygous cv+ ct+ ve+
F2 – back cross with crossveinless cut vermilion tester stock
o Expect if free recombination
Equal numbers of all 8 possible combinations
o Observe
Parentals
o cv ct ve
o +++
Single crossovers – one recombination event
o cv + +
