Heredity and Sex 73 



are invariably males, the non-barred ones being females. 

 Accordingly, the distribution of barring and non-barring 

 in the cross is sex limited. 



The barred offspring produced by a cross between 

 barred Plymouth Rocks and a non-barred breed, whether 

 those offspring are males or females, prove to be heterozy- 

 gous in barring, as we should expect, the barring factor 

 having been received only from one parent, the barred one. 

 Further, the non-barred offspring produced by a barred 

 Rock female crossed with a non-barred breed, do not trans- 

 mit barring, hence they are pure recessives as regards 

 barring. Hence, also, we are forced to conclude, as already 

 suggested, the female of the pure barred Rock breed is 

 heterozygous as regards barring, and transmits the character 

 only to her male offspring, her female offspring (if the 

 father is non-barred), neither being themselves barred nor 

 being able to transmit barring. ' 



A pure Plymouth Rock race breeds true to barring 

 merely because all its males are pure, for the females are 

 not pure. This is shown by the following experiment. 

 If a heterozygous barred male, produced by a cross between 

 a Rock and a non-barred breed, is crossed with barred 

 females, either those of a pure Rock race or those produced 

 by a cross, the result is the same. The male offspring are 

 all barred; the females, half of them barred, half non- 

 barred. This result shows that all barred females alike 

 are heterozygous in barring. 



Sex-limited inheritance such as this finds at the present 

 time its most probable explanation in the existence in the 

 egg of an extra or plus element never found in the sperm, 

 this element pairing with the sex-limited character in the 

 reduction division. Thus, in the barred Rock, calling 



