510 
L. DONCASTER. 
and their absence as a and b, the sex-factor as and the 
absence of sex-factor by a dash ( — ). 
Offspring Fi Male Xab— XAB Xab Female 
Gametes of F, - Xab nX AB XAb XaB nXab 
The number n, by which the combinations AB or ab out- 
number aB and Ab^ varies in the case of different characters 
from quite small numbers to 100 or more. Xow Morgan 
supposes that in the synapsis stage of the first-cross female 
(Fj), the chromosomes XAB and Xab pair together, and 
become twisted round one another as was mentioned earlier, 
and that in subsequent separation it is possible for A to 
exchange places with a and B with b, and that the frequency 
of this interchange crossing over will depend on the 
positions of the units representing A and B in the elongated 
chromosomed In the male, no such crossing over is 
possible, since the chromosome XAB or Xab has no similar 
mate. 
There is one difficulty, however, which makes this 
crossing-over^^ hypothesis doubtful, in spite of its beautiful 
simplicity. It is that exceptions to sex-limited transmission 
occur in almost all the known cases. Morgan himself has 
recorded some in Dros ophila, but prefers to regard them 
as due to experimental error. In other cases, however, they 
are indubitable. If sex-limited transmission were due to the 
existence in an unpaired chromosome of the units determining 
the characters, those characters must always without excep- 
tion be transmitted from the male to the female, or, in the 
Lepidoptera-bird group, from the female to the male. Excep- 
tions, however, always occur, with greater or less frequency 
^ This idea has been worked out in some detail in the case of 
Drosophila by A. H. Stnrtevant, ‘ Jonrn, Exp. Zool.,’ xiv, 1913, p. 43. 
Parents 
Male XAB - x Xab Xab Female 
