Introduction to Genetics (BIOL0003) Notes - Further Chromosome Mapping
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Course
Introduction to Genetics (BIOL0003)
Institution
University College London (UCL)
Explore the Introduction to Genetics (BIOL0003) module at University College London. This document delves into further chromosome mapping, unraveling complexities such as human haplotypes, linkage, and three-point mapping. Please note that these materials are intended for personal use only and shou...
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 + +
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