INHERITANCE OF SEX AND RELATED PHENOMENA 209 
is not certainly more frequent than in Drosophila. There appears at 
present to be good reason for accepting the explanation of non-disjunction 
for these exceptional cases, although Doncaster has advanced the sugges- 
tion that, if the sex-differentiator be assumed to occupy a definite locus in 
the Z-chromosome, then, if the Z-chromosome divides in such a way that, 
the factor / is separated from the sex factor, exceptions will be produced. 
This case has not yet been worked out as carefully as has that in Droso- 
phila, but it presents so many close analogies that the possible interpreta- 
tion is fairly clear. 
Of forms showing the WZ type of sex inheritance a number are known. 
Moths and butterflies appear to exhibit this type universally, and such 
birds as have been investigated are all of the WZ type. A familiar 
example is that displayed by the barred pattern factor in poultry. When 
black hens are mated to barred cocks, F; consists of barred hens and 
barred cocks and F2 of 2 barred cocks: 1 barred hen:1 black hen. The 
reciprocal cross barred hen by black cock gives in F; black hens and barred 
cocks, and in F, 1 barred cock: 1 black cock : 1 barred hen : 1 black hen. 
These are the relations which Pearl and Surface have demonstrated for 
crosses between the Plymouth Rock fowl and the Cornish Indian Game. 
The relations are exactly like those in the crosses of grossulariata and 
lacticolor, to diagram them it is merely necessary to substitute barred for 
grossulariata and black for lacticolor. A number of other characters in 
birds display the WZ type of sex-linked inheritance. Red-eye color in 
canaries behaves like lacticolor in Abraxas when contrasted with black- 
eye color, but exceptions seem to be unusually numerous. In pigeons a 
number of factors are known to be sex-linked. Thus in turtledoves 
normal color is sex-linked when contrasted with white; and in the 
domestic pigeon the factor for intense coloration is sex-linked. In the 
fowl Bateson and Punnett have shown that an inhibiting factor for 
silky pigmentation is sex-linked, and Pearl has demonstrated the existence 
of a sex-linked factor for high egg production. The latter case because 
of its economical importance will be given full treatment in another place. 
Besides these there are many other suspected cases, but they all occur 
either in moths and butterflies or in birds. 
Finally it remains to call attention to another analogy between the 
XY and WZ types of sex-inheritance. It is a fact firmly established by 
abundant experimentation that no crossing-over takes place in the male 
of Drosophila. Not enough evidence has yet been obtained in other 
forms to indicate whether the lack of crossing-over is a general phenom- 
enon in males that display the XY type of sex-inheritance, but it is 
highly probable that such is the case. In the silkworm moth which 
may be assumed to follow the WZ type of sex-inheritance, Tanaka has 
studied very thoroughly the linkage relations exhibited by a system of 
14 
Digitized by Microsoft® 
