Introduction to Genetics (BIOL0003) Notes - Further Chromosome Mapping

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 + +