14 
SIDNEY I. KORNHAUSER 
6. SECONDARY SEXUAL CHARACTERISTICS AND HORMONES 
The primary difference between the sexes is in the formation of 
gametes. The female is an egg producer, the male a sperm pro- 
ducer. In many animals, especially invertebrates, it is very 
difficult to distinguish males from females without first examining 
the gonads. On the other hand there is no lack of forms in which 
one can with ease distinguish the sexes by external appearances. 
Sometimes this sexual dimorphism extends to all parts of the 
organism. Compare the minute male of Bonellia with the female, 
hundreds of times its size, for example. 
Female Carrier 
X X 
Color-blind Male 
X Y 
Gametes and Combinations 
F* 
( 1 ) ( 2 ) 
Females 
XX XX 
Carrier Color-blind 
( 3 ) ( 4 ) 
Males 
X y X Y 
Normal Color-blind 
Fig. 6. Diagram Showing Sex-Linked Inheritance in Man 
An underscored X indicates the presence of the gene for normal color vision; 
because this is dominant over its allelomorph, color-blindness, any individual 
with an underscored X has normal color vision. 
Very often, however, the dimorphism is confined, first, to the 
genitalia or to accessory apparatus used in copulation, oviposi- 
tion or rearing of the young, and, second, to extragenital charac- 
teristics not associated directly with reproduction, color and 
ornaments and the like. Both of these classes are, however, 
secondary to gamete production. In mammals and birds these 
so called secondary sexual characteristics are found to be largely 
dependent for their proper development on the normal presence 
and activity of the gonad. The castration of young male mam- 
mals results in individuals lacking in many ways the attributes 
