180 PHYSICAL BASIS OF HEREDITY 



are inbred, there are four F2 classes with sex taken into 

 account in the proportion of 1:1:1:1; or ignoring sex, 

 1 barred to 1 black. The barred and the black races 

 differ by one factor difference (Fig. 79), viz., barred Z^ 

 and its normal recessive allelomorph Ti". This seems to 

 mean that the Barred Plymouth Rocks is a black race 

 with an additional dominant factor for barring. The 

 Black Langshan is the same black race but without the 

 barring factor. 



Until quite recently no cases of crossing over had been 

 observed in forms having the Abraxas type of sex-linked 

 inheritance, for, except in one or two cases in poultry, 

 only a single pair of sex-linked genes were known, and two 

 at least must be studied together in order to demonstrate 

 linkage. Goodale has recently studied two sex-linked 

 characters in poultry, and states that crossing over occurs 

 in the male, but whether or not in the female is not stated. 



Sex-detekmination and Natural Paethenogenesis 

 Variations in the ordinary sex-determining mechanism 

 account in some cases for the normal output of males and 

 females produced by parthenogenesis, and determine the 

 exceptional sex-ratios of such species. The honey bee 

 furnishes the best known example. The queen comes 

 from a fertilized egg, and has therefore the double (2iV) 

 number of chromosomes. Her eggs give off two polar 

 bodies, hence have the reduced, or single number of 

 chromosomes. Any ^gg that is not fertilized develops 

 parthenogenetically into a male. If there are two X-chro- 

 mosomes in the bee, as in some of the otjier insects, the 

 egg is expected to contain only one of them after the 

 extrusion of the polar bodies. Hence, if it develops with- 

 out doubling its chromosomes, it should give rise to a 

 male. That the male has the single number of chromo- 

 somes is also borne out by the evidence from a peculiarity 

 of the first spermatocyte division in which the cytoplasm 

 divides, but the chromosomes do not separate into two 



