18 
BULLETIN 1468, U. S. DEPARTMENT OF AGRICULTURE 
separately. In all three lots the variability between the base and 
tip samples was less than that of the individual plants. The data 
are included in Table 10 (entries 10-12). 
Table 11. — Comparison of variability of crossing over in a population' with that 
of samples from single individuals in the same population 
Standard deviations corrected for size of 
population 
Progeny 
5 samples 
from 1 plant 
4 samples 
from 1 plant 
Entire pop- 
ulation, 37 
plants 
Dh 416L3L1C5L4-1-L23 . 
2. 78±0. 59 
4. 50±1. 07 
2. 82±0. 22 
In most progenies, therefore, there seems to be no question that the 
variability in crossover rate for the population as a whole is greater 
than that found in samples of gametes from individual plants. Ex- 
ceptions have been noted, but in these cases the variability of the 
crossover ratio was relatively low. 
CROSSING OVER LESS VARIABLE IN SAMPLES FROM DIFFERENT PARTS OF THE 
SAME MALE INFLORESCENCE THAN IN SAMPLES FROM SEVERAL INFLO- 
RESCENCES 
Samples of pollen were collected separately from several parts of 
a maize tassel and applied to the silks of double-recessive plants. 
The data are given in Table 12. Unfortunately, several of the 
pollinations resulted in very few seeds, thus precluding any very 
exact determinations of differences in rate of crossing over between 
the sections of the inflorescence, though the greatest difference is 
10.4 per cent. The material can be utilized to compare the varia- 
bility of the crossover rate with that expected if the several pollina- 
tions were samples of the same population and also with that ob- 
tained in the progeny of which this plant was a single representative. 
The standard devi ation c orrected for size of population (o- 50 ) is 
4.73 ±0.92 and the \fj>q/H is 6.52, the crossover ratio being no more 
variable than is to be expected in samples of this size. The <r 50 
for the male gametes of the progeny is 8.70 ±0.71, from which the 
o- 50 of this plant departs by 3.97±1.16. Clearly, in this progeny 
the variability of crossing over, as measured in several parts of a 
single inflorescence, is less than that obtained with single measures 
of several plants. 
Table 12. — Crossing over in different parts of the tassel of a 
single maize plant 
Parts of tassel 
Number 
of seeds 
Percentage of 
crossovers 
395 
305 
257 
64 
176 
14 
27. ldbl. 50 
Uppermost tassel branch -- 
22. 6±1. 61 
Branch 2 
29. 6±1. 92 
Branch 4 ... 
26. 6±3. 72 
Branch 6 
31. 8±2. 36 
Branch 10 
21. 4±7. 40 
Mean 
27. 09±1. 16 
C50 _ 
4. 73± . 92 
-y/pq/H. 
6.52 
0"s» for progeny of which this is one plant 
8.70± .71 
