Lecture 7 Lecture 7
Lecture 7Lecture 7
Lecture 7
Last time we discussed how to measure the distance between two genes on the X
chromosome. To do this we used the trick of looking only at male progeny so the
genotype of the X chromosome could be scored directly since these flies only carry one
copy of the X. For autosomes, we can have the same ability to score all recombinant
classes by crossing to a homozygous recessive individual. This is known as a test-cross.
Consider the recessive traits vestigial wings and short bristles that are specified by two
different genes on the same autosome (non-sex chromosome).
+ ++ +
+ ++ +
+ +
vg shvg sh
vg shvg sh
vg sh xOO vg shvg sh
vg shvg sh
vg sh + ++ +
+ ++ +
+ +
↓
vg shvg sh
vg shvg sh
vg sh
All F1: + ++ +
+ ++ +
+ +
The F1 flies are heterozygous for both genes so we are in position to see how
oftencrossovers between these chromosomes occur in meiosis by doing a test-cross.
O vg shvg sh
vg shvg sh
vg sh vg shvg sh
vg shvg sh
vg sh
x O
+ ++ +
+ ++ +
+ + vg shvg sh
vg shvg sh
vg sh
↓
(Note that the progeny vg shvg sh
vg shvg sh
vg sh 458
have distinct phenotypes) vg shvg sh
vg shvg sh
vg sh
vgvg
vgvg
vg shsh
shsh
sh 442
+ ++ +
+ ++ +
+ +
vg +vg +
vg +vg +
vg +
vg shvg sh
vg shvg sh
vg sh 47
(crossover classes)
++
++
+ shsh
shsh
sh 53
vg shvg sh
vg shvg sh
vg sh
100
The distance between vgvg
vgvg
vg and shsh
shsh
sh = 100 x = 10 cM
1000
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