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 
soon. 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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