HEREDITY AND SEX 



breed the feathers are marked with alternate 

 bars of darker and lighter black. Pure barred 

 Rocks breed true, but when crossed with other 

 breeds, the male proves to be homozygous, the 

 female, heterozygous in barring. For the male 

 Rock crossed with a non-barred breed produces 

 only barred offspring in both sexes, but the 

 female Rock crossed with the same non-barred 

 breed produces offspring approximately half 

 of which are barred, the other half being non- 

 barred. Further, the barred individuals in this 

 cross 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 heterozygous in barring, 

 as we should expect, the barring factor having 

 been received only from one parent, the barred 

 one. Further, the non-barred offspring pro- 

 duced by a barred Rock female crossed with 

 a non-barred breed, do not transmit barring, 

 hence they are pure recessives as regards bar- 



171 



