114 GENETICS IN RELATION TO AGRICULTURE 
mated to black curved males, according to the same investigators, 
gave the results tabulated in the last two columns in Table XXIII. Here 
the percentage of crossing-over amounted to 22.74 per cent., a value 
substantially in agreement with the results of the reverse factor tests. 
Taste XXIII.—Crossinc-oveER BETWEEN B AND C, IN DROSOPHILA 
Black curved o& (bev) (bev) mated to 
Phenotype Gray normal 2 (BCv») (bev) Gray normal 2 (Bev) (bC») 
Non- Non- 
eeoorweee Cross-overs pa pe ta | Cross-overs 
= 
Gray normal ersacme cee er 610 Hoo) Slimesees 644 
GrayACuryed\ esr cn cae acre ee 184 2,292 
Blacknormal Sox cace se ase nacre ae 226 2,148 | 
Black curved...............-. 652 eee. th peioeroe | 663 
otal sits a cae ace wee 1,262 410 4,440 | 1,307 
Percentage of crossing-over.... . 24.04 | 22.74 
No Crossing-over in the Male.—The above results show clearly that 
crossing-over in the female between the loci B and C, of the second chro- 
mosome results in the production of approximately 23 per cent. of cross- 
over gametes irrespective of the particular combination of the factors 
concerned. Sturtevant and Bridges, however, have shown that there is 
no crossing-over in the male so that males of the genetic constitution 
(BC,)(bc.) produce only two types of sperm in the ratio 1(BC,) :1(bc,) 
and males of the genetic constitution (Bc,)(bC,) produce sperms in the 
ratio, 1(Bc,) :1(bC»). 
It is not known just what the absence of crossing-over in the male 
depends upon. In the case of factors in the X-chromosome or first 
group, crossing-over would involve exchange of chromatin material be- 
tween the X- and Y-chromosomes. Since these differ strikingly it is 
not surprising that interchange of chromatin does not take place between 
these chromosomes for it is difficult to see how the difference could be 
preserved if crossing-over should occur. But the other chromosomes are 
alike in both sexes, nevertheless no matter how high the percentage of 
crossing-over in the female, none whatever has been observed in the male. 
This has been found true for factors lying in the third group as well as 
for those lying in the second group, and it is without doubt a general 
phenomenon. 
The knowledge that no crossing-over occurs in the male has often 
been turned to advantage in experimental work. When gray curved 
flies, (Bc,)(Bc,) are mated to black normal, (bC,)(bC,), the Fy flies are 
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