336 Recent Advances hi the Study of Heredity. 
In the case of both Bryonia and Abraxas the theory which 
gives the simplest explanation of the facts is that the female is a 
heterozygote (femaleness being dominant) and the male a homo¬ 
zygous recessive. This conclusion is supported by the facts of sex- 
limited heredity. The nature of this phenomenon can most easily 
be made clear by describing a concrete instance in which it is 
manifested. 
In the Dorset Horned sheep the horns are well developed in 
both sexes. In the Suffolks horns are not developed either in the 
rams or the ewes. When individuals of these two races were 
crossed, it was found that the Fj male lambs had horns of fair size 
whilst the Fj^ ewes were hornless. The horned character may 
therefore be said to be dominant in males but recessive in females. 
We are probably nearer the truth if we say that there is something 
in the female which suppresses the character—which limits the 
apparent inheritance of that character to the male sex—and that 
the male lacks this something. 
The same phenomena are exhibited by the inheritance of 
colour-blindness. About 4% of the males in European countries are 
colour-blind ; whereas less than '5% females are. Colour-blindness 
is transmitted chiefly to sons, very rarely to daughters ; but the non- 
colour-blind daughters frequently transmit it to their sons again. 
There appears, therefore, to be some factor in the organization of the 
female which suppresses the colour-blind character, just as it 
suppressed the horned character in the sheep. 
It follows that women can transmit colour-blindness without 
having it; whilst men cannot. 
It is argued that this proof that the females possess this 
“ suppressing ” factor which is absent from the male, supports the 
theory that femaleness consists in the presence of something whilst 
maleness is determined by the absence of that something. Female- 
ness, therefore, is dominant; whilst male^^ss is recessive. But “ a 
female ” is a heterozygote, half of her gametes bearing maleness, 
and half femaleness ; whilst a “ male ” is a homozygous recessive. 
The evidence which we have considered so far is consistent 
with the theory that the female is the heterozygote. In the next 
lecture we shall deal with facts which, whilst they support the 
theory that one sex is heterozygous and the other homozygous, 
point to the male as the heterozygote and the female as the 
homozygote. 
