36 
BULLETIN 1468, U. S. DEPARTMENT OF AGRICULTURE 
may be examined for the effect of the Su factor. It will be seen 
that in none of the comparisons is the difference statistically signifi- 
cant. It should be noted, however, that in three comparisons where 
the female is the heterozygous parent there is a higher crossing over 
in the ears not segregating for Su. 
Table 23. — Comparison of percentages of crossing over in- plants heterozygous 
for Su with those in plants homozygous dominant for this factor 
Heterozygous for Su 
Homozygous for Su 
Difference 
(homozygous 
—heterozy- 
gous) 
Nature of cross 
Num- 
ber 
of 
ears 
Num- 
ber 
of 
seeds 
Percentage 
of 
crossovers 
Num- 
ber 
of 
ears 
Num- 
ber 
of 
seeds 
Percentage 
of 
crossovers 
DIE 
Female gametes: 
50 per cent white 
10 
12 
11 
9 
5 
5,637 
4,576 
4,426 
2,451 
1,873 
21. 21±1. 20 
16. lftfcl. 18 
20. 61±1. 14 
26. 99±2. 30 
27. 72±1. 54 
10 
22 
5 
12 
10 
4,737 
6,642 
1,215 
3, 432 
3,781 
22. 04±0. 57 
18. 31± . 76 
26. 49±1. 63 
25. 62± . 95 
26.24± .98 
0. 83±1. 33 
2. 12=fcl. 40 
5.88±1.99 
-1.37±2.47 
-1.48±1.79 
0.62 
62.5 white.. 
1.52 
75.0 per cent white 
Male gametes: 
50.0 per cent white 
62.5 per cent white 
2.95 
.55 
.83 
RATE OF CROSSING OVER DIFFERS IN THE TWO SEXES 
One of the most striking facts concerning the association of the 
chromosomes with the transmission of characters is that in many 
forms the chromosomes behave very differently in the formation of 
male and female gametes. In the male of Drosophila when the 
segregation of characters takes place in the formation of gametes 
the chromosomes behave as units. All the characters of any given 
chromosome are transmitted together as they were received from the 
parents. It would seem not unreasonable that this difference should 
in some way result from cytological differences in the genesis of the 
microspores and megaspores, but there are other forms in which the 
reverse is true, and it is the female in which there is no crossing 
over. In still other forms, especially in many monoecious plants, 
there appears to be an equal degree of freedom of crossing over in the 
two sexes. So long as difference in rate of crossing over remains an 
isolated fact unrelated to other cytological or genetic phenomenon, 
it will be necessary to grope for bearings and approach the problem 
from as many angles as possible. 
In Drosophila, where there is absolutely no crossing over in the 
male, it seems difficult to proceed by other than cytological methods, 
but in species where the crossing over takes place in both sexes but 
with different degrees of freedom it should be possible to determine at 
least some of the factors that influence or are associated with the 
diversity. 
Individual diversity in the rate of crossing over is so great that 
little reliance can be placed on comparisons of the male and female 
rate of crossing over unless the rate in both sexes is measured in the 
same individuals. Furthermore, there is some evidence that the appar- 
ent rate of crossing over is influenced by the double recessive parent. 
For this reason the material examined in this connection has been 
restricted to reciprocal crosses of double-heterozygous and double- 
recessive individuals. 
