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EEMB 129  LECTURE 8 2010
1
I. Linkage (Chapter 5)
X2 Test
Tetrad Analysis
X
2
Test
When testing for linkage, how different from 50% do the recombinant classes need to
be before we conclude that two genes are linked?
X
2
= sum of the (Obs  Exp)
2
Exp
As the Obs approaches the value of the expected, then the X
2
value is small
If the Obs is fairly different than the expected, then the X
2
value will be larger
AaBb
x
aabb
Expected
if
independently
OBS
assorting
O  E
(O  E)
2
(OE)
2
/E
AaBb (AB)
18
12.5
5.5
30.25
2.42
Aabb (Ab)
7
12.5
5.5
30.25
2.42
aaBb (aB)
10
12.5
2.5
6.25
0.5
aabb (ab)
15
12.5
2.5
6.25
0.5
50
50
∑
5.84
degrees of freedom:
# of freely varying parameters. If you examine 50 individuals, any three
classes can vary freely, but once you specify how many individuals are in three classes, the
last class is fixed.
for 3 degrees of freedom, we can then look at a X
2
table
Now look at a Chisquare table:
Find the row for the degrees of freedom
Look across to see where your X
2
value lies
Look up to the probability to see between which values your value lies
X
2
, df = 3
2.37
<
5.84
<
6.25
corresponding
P
0.50
p
0.10
Therefore IF the genes truly were independent (the null hypothesis), THEN you
would have between 1050% chance of obtaining a X
2
value of 5.84
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View Full DocumentEEMB 129  LECTURE 8 2010
2
Tetrad Analysis:
There are a number of questions about meiosis that cannot be answered by traditional
genetic methods (i.e., with diploid organisms such as fruit flies).
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 Spring '09
 AJNARIVERA

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