Recent Advances in the Study of Heredity. 331 
closer harmony with the mode of inheritance of sexual characters, 
is that the members of one sex are heterozygous as regards their 
sex ; whilst those of the other are pure recessives. This theory 
provides a very simple explanation of the production of the two 
sexes in equal numbers ; the result of the mating DR x RR being, 
of course, DR and RR in equal numbers. The facts which have 
been, brought to light by the study of the inheritance of Mendelian 
and of sexual characters and by the study of the cytological 
phenomena in certain insects all strongly support this view; but 
the evidence derived from these several sources is strangely conflicting. 
Our conclusion therefore is that one sex is a heterozygote and the 
other is a pure recessive; but the evidence on the question which 
is the heterozygote and which is the recessive is not at present 
conclusive. 
The conclusion reached by the experimental treatment of this 
problem by means of breeding experiments is that the female is the 
heterozygote and the male the recessive. Let us consider two 
classical cases which support this view ; and let us deal first with 
Correns’ crosses of Bryonia. Correns made a number of crosses 
between Bryonia dioica and Bryonia alba, which is monoecious. 
B. dioica 5 X B. alba gave “ F x females, with or (usually) 
without occasional male flowers. The observed numbers from this 
mating were 989 females, with 2 males which must be regarded as 
exceptional.” 
B. alba (monoecious) used as a female X dioica $ gave an F t 
consisting of males and females in approximately equal numbers. 
Correns offered an explanation of these phenomena which involved 
the assumption that maleness was dominant, that is to say that the 
male is heterozygous and the female pure recessive. The zygotic 
constitution of the male is $ 5 , and of the female ? ? . The male and 
female cells of alba are supposed to be undifferentiated in respect 
of sex. Correns’ theory may be summarised as follows :— 
dioica 5 is J ? . 
dioica $ is $ ? . 
alba ? is £ . 
alba is also $. 
Thus when normal dioica mate inter se the mating and its 
result may be represented as follows 
dioica ? x dioica $ gives rise to females ? 5 and males 5 $ 
The first cross we considered would be dioica ? x alba $ 
giving rise to all females of the form 5 £ . 
