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 I 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, FI consists of barred hens and 

 barred cocks and f\ 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 Fz 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 

 grossulanata and black for laaicolor. 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 



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