58 BULLETIN 1468, U. S. DEPARTMENT OF AGRICULTURE 
The coefficients of correlation of a series of characters for 47 pairs 
of reciprocal back-crossed ears are given in Table 35. Most of the 
characters included are self-explanatory. With the exception of 
" difference in crossover," the characters are repeated in the recipro- 
cal ears. Difference in crossover is the percentage of crossing over 
in the female gametes as measured on the double-heterozygous plant 
minus the percentage of crossing over in the male gametes measured 
on the ear produced on the double-recessive plant. The character 
" Base without seeds " is the distance from the base of the cob to the 
first seed borne on the ear. This character was included as being an 
indication of imperfect pollination that might be associated with 
weak pollen or ovules. Coefficients that exceed three times the prob- 
able error have been printed in italic type. These are few in number 
and for the most part are of a physical nature. The correlation of 
0.487±0.075 between the crossing over in male and female gametes, 
when compared with the absence of significant correlations between 
the percentages of white or waxy in the two sexes, is strong evidence 
that there is markedly more individual diversity in the rate of cross- 
ing over than there is in Mendelian ratios. 
In all comparisons involving the crossing over in both sexes, it 
was necessary to confine the material to reciprocal crosses, Ad- 
ditional material was available, however, for the characters con- 
fined to single ears. Table 36 gives the correlation coefficients for 
characters of back-crossed ears grown from seeds that were double 
heterozygous and double homozygous, respectively. To the char- 
acters included in the table of reciprocals there has been added 
" Number of seeds per centimeter " and " Mean weight of seeds." 
The first is the total number of seeds divided by the length of the 
ear expressed in centimeters. 
The only character significantly correlated with crossing over is 
the number of seeds in the ears, measuring the crossing over in the 
male, 0.278 ±0.059. This was mentioned in connection with the ex- 
planation of the earlier data. The corresponding correlation in 
the ears produced on double-heterozygous plants is 0.042 ± 0.073. 
These coefficients indicate that in this progeny at least a differential 
death rate is operative on the male but not on the female gametes. 
If the vigor of the homozygous plant on which the crossing over 
in the pollen is measured were affecting the survival of the cross- 
over class, it should have been apparent in the correlations between 
crossing over in the male and certain characters, namely, ear length, 
number of rows, and mean weight of seed. These coefficients, though 
insignificant, are all positive in sign. On the other hand, the absence 
of any correlation between crossing over in the female and the number 
of seeds on the ear measuring the crossing over is in accord with the 
correlations with the other measures of vigor, two of which are nega- 
tive in sign, that with mean weight of seed being nearly three times 
the probable error. 
