Peer tutoring, must have gotten an Aor anA- in the class, must have taken the course at
Genetic Mapping II
• Centromere mapping wt tetrads
• Chi square in linkage analysis
• physical vs recombinant maps
in breeding, often trying to introduce as single trait into a line, not other traits. (ex: disease
Might take 15-20 generations to keep on crossing over and over again, but if you have markers on
either side, you can speed up the process. This use of markers on the side is a multi-million dollar
industry these days.
Takes a long time to build up maps, so want to do it with multiple traits. Today, we're going to 3
Drosophila wt vermillion eyes (v), cv (cross veinless wings) ct (cut wings )
Parental gametes = v+ cv ct, x v cv+, ct+
8 different gamete types for the children. Your working wt a second parent who doesnt mask the
Is there evidence of linkage?
The evidence of linkage would be more parental type gametes than recombinant. And we do have
an abundance of parent type, suggesting evidence of linkage.
3 traits, 2 parentals, 6 recombinant types
To find out how far the genes are from each other, calculate the percent of that particular
recombinant/total recombinants.....for v and ct = 13.2%
For cv and ct, 6.42%
For v and cv, 18.5 %
Using these percentages, we have a map units.
v----------13.2 map units...........ct......6.4 m u........................cv
IT'S NOTALWAYADDITIVE LIKE ITAPPEARS TO BE. 19.6 is not equal to 18.5 What did the F1 parent look like, well there was the possibility that there was a double crossover,
switching the order of the middle genes
add in the last two things in the first column twice as R+R, which will account for the potential of a
double crossover, and you will get the 19.6 expected
YOU COULD SEEA 3 POINT TESTCROSS ONAN EXAM, BUT NOTAFOUR OF FIVE
POINT, AS THE 3 POINTALONE IS DIFFICULTASALL HELL
Another little nuance: Phenomenon of interference. ( calculating the expected probability of double
Multiply the probability of the crossovers between 2 of the pairs (prodct rule) and get the expected
Map unit = probability of a cross over, so....
Probability of a crossover between v and ct+ = 0.132
Probability of a crossover between ct+ and cv+ =