Example 2
Strickberger. pg 317
Cross AaBb x AaBb and expect F2
ratio of 9:3:3:1 with complete dominance gene action at each locus and
no linkage.
| |
A_B_ |
A_bb |
aaB_ |
aabb |
Total |
| observed |
180 |
30 |
60 |
10 |
280 |
| expected |
157.5 |
52.5 |
52.5 |
17.5 |
280 |
| X2 = |
(180-157.5)2 |
= |
(30-52.5)2 |
+ |
(60-52.5)2 |
+ |
(10-17.5)2 |
| 157.5 |
52.5 |
52.5 |
17.5 |
= 17.14 w/3df which is significant
There are three possible explanations from the significant deviation
from the expected 9:3:3:1 ratio. Explanations include: Deviation from
3:1 ratio at 'A' locus; deviation from 3:1 ratio at 'B' locus; lack
of independent assortment between the two genes due to linkage.
A Locus
| |
A_ |
aa |
Total |
| observed |
180+30=210 |
60+10=70 |
280 |
| expected |
210 |
70 |
280 |
X2
= 0 w/1df, not significant accept Ho:3A_:1aa.
B Locus
| |
B_ |
bb |
Total |
| observed |
180+60=240 |
30+10=40 |
280 |
| expected |
210 |
70 |
280 |
X2
= 17.14 w/1df, significant so reject Ho:3B_:1bb
Linkage
Contingency X2
test w/1df
| |
B_ |
bb |
|
| A_ |
180 |
30 |
210 |
| aa |
60 |
10 |
70 |
| |
240 |
40 |
280 |
| X2 = |
[|(180)(10)-(30)(60)| - ½(280)]2 x 280 |
|
|
| (240)(40)(210)(70) |
|
|
| = |
[|0 - ½(280)|]2 x 280 |
= 0 |
|
|
|
|
| (240)(40)(210)(70) |
|
|
|
|
So fail to reject Ho: independence between A and B
Conclusion
There is no distortion in the segregation ratio for the 'A' locus.
There is a distorted segregation ratio for the 'B' locus. There is no
indication that the two loci are not segregating independently.
** "Even distorted segregation ratios will not affect the test
for linkage." This is because the three tests are orthogonal. We
will show this.
