Negative interference – more double cross overs then expected (cross over in
one region promotes cross overs in another region)
Positive interference (usually) – fewer double cross overs than expected (one
cross-over region inhibits cross-over sin another region)
3 point test cross. Which gene is in the middle?
ABC 450 P
abc 440 P
Abc 4 DCO
aBC 6 DCO
ABc 25 SCO (a +c)
abC 30 SCO (a+c)
aBc 25 SCO (a+b)
AbC 20 SCO (a+b)
o Compare your parental and DCO groups to get middle gene. Which
What is A-C map distance?
o = % recombinants A-C
25+30+4+6 / 1000 x 100 = 6.5 m.u
What is A-B map distance?
o =% recombinants A-b
25+20+4+6/1000 x 100 – 5.5 m.u
What is the total distance?
o = 6.5+5.5
o 1- observed DCO/ expected DCO
o 1- (4+6) / [(0.55)(0.065) x 1000]
Multiply individual probabilities to get expected DCO
Cross individual homozygous recessive at a and b to an individual
homozygous recessive at c. Then test cross the resulting trihybrid. What is
the probability of the abc phenotype in the progeny?
o aabbCC x AABBcc
baC/baC x BAc/BAc
o We need a cross over at a and c to get progeny of abc.
P(SCO between a and c) x p(no cross over for a and b) 6.5 map distance between a and c (needs to happen
twice to get it, so half it) and multiply for the chance of
the other cross over not happening
o % chance is 0.055 minus from 1 to get chance of
=(1/2)0.065 x (1-.055)
o 0.0307 = 3% chance
A-B distance= 20 mu