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More Chapter 5
Linkage
Quantitation
Gene A
Gene B
Gene C
Recombination % is proportional to distance between genes.
Single recombination
As you analyze genes that are increasingly far from each other on the same
chromosome, the observed recombination frequency approaches, but
doesn’t exceed 50% for two genes on the same chromosome. 50%
recombination is the same value that is observed for two independently
assorting genes on different chromosomes.
Here are two
genes on
different
chromosomes
showing
independent
assortment.
Semicolon
separates genes
on different
chromosomes.
Here are two
genes on the
same
chromosome but
so far apart that
they exhibit
independent
assortment.
Unlinked genes show a 50% recombination frequency. In other words,
Phenotypes segregate in a 1:1:1:1 ratio, just as Mendel described.
If an AB/ab F1 peas are selfed, and you observe 16% ab/ab F2
progeny, how many map units separate the A and B genes.
Recombinant
Parental
A B
a
A
b
10%
a
B
40%
A
B
40%
a
b
b
Recombinant
A b
a
10%
Parental
B
20% recombinants
or 20 mu = 20 cM
10% A
b
10% a
B
40% A
B
40% a
b
If A and B are unlinked, what percent aabb progeny are expected?
ab/ab
16%
1/16 or 6.25%
If A and B are 28 mu apart, what percent of the progeny
from selfing the heterozygote AB/ab would be Ab/ab?
Parental
A B 36%
a
b 36%
Recombinant
A b 14%
a
B 14%
Recombinant
14%
A
72%
28%
2 X (36% X 14%)
= 10% Ab/ab
14% A
b
14% a
B
36% A
B
36% a
b
b
14%
a
B
Parental
36%
A
B
36%
a
b
Ab/ab
5.04%
Ab/ab
5.04%
= 1 map unit= 1cm
Parental and recombinant
classes depend on past history.
F1
w+
y+
w
y
w+
y
w
y+
X
yw/y+w+
F1
• Recombination represents a change in linkage phase.
• Rf is independent of the arrangement of alleles.
y+w+/Y
X
yw+/y+w
y+w/Y
Mapping
Each gene has a
specific address or
map position
Genes that are on
different chromosomes or
that are 50 or more cM
away from each other
will show independent
assortment.
Cloning the cf gene utilized markers in the region.
Sturtevant and Morgan began mapping all of the X linked
mutations relative to each other in pairwise combinations.
w is obviously closer to y than m is. But what is linear
order of these genes?
Map more markers! Make a linkage map!
Find distance between y and m.
Units separating genes are additive.
Can draw a linear map = chromosome.
Two problems.
1. Not really
sure of y and w
gene order.
Could be
sampling error?
2. Distances are
not adding up.
2 point
crosses
Need 3 point
crosses
50map units is independent assortment. 2 genes can be so far apart that they
don’t show linkage, but can belong to a linkage group, by virtue of their distance
relative to other closer genes.