16 THE METHODS AND 



are pure in the possession of an ingredient, 

 or in not possessing it ; and the ingredients, 

 or factors, as we generally call them, are units 

 because they are so treated in the process of 

 formation of the new gametes and because 

 they come out of the process of segregation 

 in the same condition as they went in at 

 fertilisation. 



As a consequence of these facts it follows 

 that however complex may be the origin of 

 two given parents the composition of the off- 

 spring they can produce is limited. There is 

 only a limited number of types to be made 

 by the possible recombinations of the parental 

 ingredients, and the relative numbers in which 

 each type will be represented are often pre- 

 dicable by very simple arithmetical rules. 



For example, if neither parent possesses 

 a certain factor at all, then none of the off- 

 spring will have it. If either parent has two 

 doses of the factor then all the children will 



