Types of Crosses 



union of parents which closely resemble one 

 another. Now, when two unlike forms inter- 

 breed, their offspring will fall into one of six 

 classes. 



I. They may exactly resemble one parent, or 

 rather the type of one parent, for, of course, 

 they will never be exactly like either parent ; 

 they must of necessity display fluctuating varia- 

 tions. The cases in which the offspring exactly 

 resemble one parent type in all respects are com- 

 paratively few. They occur only when the 

 parents differ from one another in one, two, or 

 at the most three characters. Thus when an 

 ordinary grey mouse is crossed with a white 

 mouse the offspring are all grey, that is to say, 

 they resemble the grey parent type. Although 

 they are mongrels or hybrids, they have all the 

 appearance of pure grey mice. This is what is 

 known as unilateral inheritance. 



II. The offspring may resemble one parent 

 in some characters and the other in other 

 characters. They may have, for example, the 

 colour of one parent, the shape of the other, and 

 so on. Thus if a pure, albino, long-haired, and 

 rough-coated male guinea-pig be crossed with a 

 coloured, short-haired and smooth-coated female, 

 all the offspring are coloured, short-haired, and 

 rough- coated. That is to say, they take after 



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