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
ex­-reciprocals

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