1.
Crossing over differences in reciprocal backcrosses in corn.
In translocation 5‑9a (break in the long arm
of 5 beyond the subterminal knob and in the short arm of 9), c sh wx and v2 are in the translocated pieces of chromosomes 9 and
5 respectively. The percentages of recombination observed in reciprocal
backcrosses for plants heterozygous translocation, homozygous translocation (c
sh wx now beyond the knob in the
long arm of 5) and in standard normal plants were for
|
C + + |
: |
|
c sh wx |
|
|
|
Region 1 |
Region 2 |
Totals |
No. of |
|
T |
|
3.0 |
6.6 |
3779 |
19 |
|
+ |
3.7 |
12.5 |
5949 |
19 |
|
|
|
|
|
|
|
|
|
T |
|
4.6 |
19.4 |
6394 |
23 |
|
T |
4.3 |
23.0 |
6433 |
23 |
|
|
|
|
|
|
|
|
|
+ |
|
5.0 |
22.9 |
2336 |
-- |
|
+ |
4.2 |
27.5 |
3554 |
-- |
For
|
+ + + |
|
sh wx gl |
(this glossy is gl4 Hayes until its
relation to Sprague's No. 10 is clear).
|
|
|
Region 1 |
Region 2 |
Totals |
|
T |
|
7.1 |
6.7 |
75 |
|
+ |
14.9 |
5.3 |
455 |
|
|
|
|
|
|
|
|
+ |
|
18.7 |
8.6 |
326 |
|
+ |
24.0 |
10.6 |
303 |
The large differences in observed recombinations
appear to be limited to the sh‑wx region in plants heterozygous for T5‑9a, although the numbers
for wx‑gl in the 
test
are small. If orientation of cross-over chromatids is the explanation,
some interesting questions are raised. There may be a relation to centromere
position or to heterochromatin. Tests are under way to compare crossing
over in regions near the centromere in other chromosomes. Results for gl2-v4 in chromosome 2 are negative, but for gl‑In in chromosome 7 a limited test gave 15.2 per cent
in the cross using the heterozygote as the female and 33.2 per cent as the
male.
C. R. Burnham