Lecture 12 – Linkage Mapping/Recombination
I. Discovery of linkage
A. Unusual results from cross:
- cross P/p·L/l X P/p · L/l (Note: this P is different from P we’ve encountered previously)
P=purple
p = red
L = long pollen
l = round pollen
Phen (gen)
observed
expected
purple, long (P/– · L/—)
purple, round (P/– · l/l)
red, long (p/p · L/–)
red, round (p/p · l/l)
- so what’s going on here?
B. linkage - remember that unlinked genes show independent assortment
WwGg X WwGg

W-GW-gg
wwGwwgg
- if genes completely linked, they assort as one
AB/ab X AB/ab

AB/AB
AB/ab
ab/ab
- independent assortment applies to unlinked genes
WwGg X WwGg
W-GW-gg
wwGwwgg
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- What about linked genes?
AB/ab X AB/ab
AB/AB
AB/ab
ab/ab
C. Recombination - if
purple, round and red, long
P and L are linked, where did
come from? Answer:
- crossing-over
- gene occupies well defined site, or locus
- alleles of genes occupy corresponding locations on homologous chromosomes
2
- crossing-over occures in prophase /metaphase I
- exchange consists of breaking and rejoining of 2 chromatids with reciprocal
exchange of equal, corresponding segments
- may be multiple crossovers between homologous chromatids
- site of crossing-over is ~random, > length between loci = >probability of
crossover
D. few more points about recombination:
- recombination only results when crossover occurs between markers
- double crossover between markers is not detected
II. genetic notation/more terms
A. cis vs trans
may be represented by
1. cis (aka coupling)
2. trans (aka repulsion)
B. Parental vs. nonparental
1. parental combination
2. nonparental combination
III. Distinguishing linked from unlinked – use test cross
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A. Consider 2 examples:
AaBb X aabb


AaBb
Aabb
aaBb
aabb



CcDd X ccdd


CcDd
Ccdd
ccDd
ccdd
B. Often, considering only male progeny is~equivalent to test cross (if x-linked)
w+ = red eyes
w = white eyes
wm+/wm+ female X w+m/Y male

m+ = normal wings
m = mini wings
wm+/w+m & wm+/Y

wm+/Y (white eye, normal wing)
w+m/Y (red eye, mini wing)
wm/Y (white eye, mini wing)
w+m+/Y (red eye, normal wing)
- what if mutations is in cis?
wm/wm female X w+m+/Y male

wm/w+m+ & wm/Y

+
+
w m /w m & w +m+/Y
wm/w m & wm/Y
wm+/wm & wm/Y
w+m/wm & w+m/Y
C. Recombination is suppressed in Drosophila males
cn = cinnabar eyes
cn+ = red eyes
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bw = brown eyes
bw+ = red eyes
cn bw/+ + female X cn bw/cn bw male


+ +/cn bw
cn bw/cn bw
cn +/cn bw
+ bw/cn bw
cn bw/cn bw female X cn bw/ + + male



cn bw/+ +
cn bw/cn bw
IV. Summary
A. Two modes of reassortment
1. independent assortment
2. recombination
B. Few additional points
1. if perfectly linked
2. maximum recombination is 50% because:
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