I30 WHAT EVOLUTION IS 



white gene, fertilized by a sperm also 

 containing a white gene. This is rep- 

 resented in the diagram by the two 

 white circles above the white pig, one 

 for the egg and the other for the 

 sperm. Such an individual can pro- 

 duce only one class of germ cells, 

 namely those with white genes. As- 

 suming in this particular instance 

 that the white pig is the female, she 

 may then be described as an individ- 

 ual producing eggs all of which carry 

 the white gene. If the white member 

 of the pair is the female, the black one 

 must be the male and what has been 

 said of the white pig may be said of 

 her black mate, except that black gene 

 is to be used in place of white gene 

 and sperm cell in place of tgg cell. 



In the first descendent generation 

 all offspring, in the present instance, 

 would be the product of a white tgg 

 fertilized by a black sperm. This is 



