Sex Dclcrui'nuUiou 183 



such a chromosome. As a result lluTc arc two kinds of 

 sperms, one haU' conlaininj^^ a sex chromosome, the other 

 half containing none. At fertilization, if an egg mates 

 with a sperm having a sex chromosome the zygote con- 

 tains two, and this will develop into a female, for females 

 are characterized by two sex chromosomes. With a 

 sperm of the other type, the zygote receives only one sex 

 chromosome and must produce a male individual. As a 

 result, males and females are produced in equal numbers, 

 sex being determined by the type of sperm that enters 

 into the sex fusion. 



Certain conclusions may be drawn from this mecha- 

 nism of sex determination, which will serve to provide 

 a sharp contrast with the corresponding conclusions that 

 may be drawn from the physiological theories. 



a) The sex ratio will regularly be 50 per cent males: 

 50 per cent females. It would be rather hopeless to 

 modify this ratio by artificial means. 



h) Sex is a qualitative matter, only two conditions 



being possible, strictly male and strictly female. 



Numerous instances of the sex chromosome mechanism have 

 been discovered in the animal kingdom. Details ditler in the 

 different cases, but the essential mechanism remains the same. 

 In addition to the type of case described above, where the male- 

 has only one member of the sex chromosome pair, there are in 

 general three other possibilities. The male may have one largo 

 chromosome (similar to the pair in the female) paired with a small 

 one; the male may have two sex chromosomes of ai^proximately 

 the same size but different in shape; or the male may have two 

 sex chromosomes which are morphologically itientical, but physi- 

 ologically different in their intluence on sex. In all of these cases 

 the fundamental mechanism remains the same, the male being 

 heterozygous for sex, so that two types of six^rms are protluced in 

 equal numbers, and the sex of the olYspring depends upon which 



