A total of 385 translocations now have been analyzed cytologically to determine the position of the breaks. These data are summarized in table 1 by chromosome arms. The translocations in group 1 include those listed in last year's report with a few additions. Those listed in group 2 are recently acquired stocks.

Table 1. Frequency of chromosome breaks in each
of the 20 maize chromosome arms

Group 1				Group 2
Chromosome	Arm	Breaks	Breaks per u	Chromosome	Arm	Breaks	Breaks per u
		No.	No.			No.	No.
1	S	21.5	.60	1	S	18	.50
1	L	31.5	.68	1	L	25	.54
2	S	14.	.47	2	S	12	.41
2	L	42.	1.14	2	L	15	.41
3	S	11.5	.56	3	S	13	.63
3	L	33.5	.81	3	L	19	.46
4	S	21.	.97	4	S	14	.62
4	L	21.	.58	4	L	20	.55
5	S	24.5	.90	5	S	18	.66
5	L	31.5	.97	5	L	27	.83
6	S	9.5	.80	6	S	11	.92
6	L	33.5	.91	6	L	21	.57
7	S	8.	.65	7	S	8	.65
7	L	26.	.76	7	L	22	.64
8	S	13.	1.15	8	S	15	1.33
8	L	27.	.75	8	L	27	.75
9	S	6.	.39	9	S	13	.85
9	L	32.	1.14	9	L	20	.71
10	S	8.	.82	10	S	5	.51
10	L	25.	.92	10	L	7	.26
Total		440	Mean .796	Total		330	Mean .597

To analyze these data further so as to show a variation in density of breakage points along the chromosome arm, a second table has been prepared.

Table 2. Frequency of breaks, expressed in per cent, for the sum of each 5 micron section of the 20 chromosome arms beginning at the fibre attachment

Group	1st 5 u	2nd 5 u	3rd 5 u	4th 5 u	5th 5 u	6th 5 u	7th 5 u	8th 5 u
1	20.8	21.3	14 2	12.4	12.7	11.1	5.4	2.2
2	19.7	22.6	9.7	10.9	10.7	14.3	6.7	5.5

A. E. Longley