980 



LIFE : OUTLINES OF GENERAL BIOLOGY 



the germ-cells undergo the process of maturation, the members of 

 each pair of genes separate ; and the result is that each of the result- 

 ing germ-cells has only one of each pair of genes, some having the 



di 



d2 



a b bi 



Fig. 167. 



The Chromosomes of the Germ-Cells become closely associated in pairs, a 

 "synaptic" apposition, as in a. They are living structures and may move 

 crosswise on one another, and in different ways, as suggested in ai as 

 compared with b. Part of the one chromosome may cross over into the 

 other. But when the members of a synaptic pair become definitely 

 separate again, the results may be different in different cases, as is 

 suggested by the contrast between a2 and 61. This is one of the ways 

 in which variations may arise. After Morgan . 



one and some the other. In the subsequent fertilisation, there is an 

 assortment of the genes, so that a fertilised egg-cell may have two 

 of the same type or two of different types. Yet it must be noticed 



Fig. 168. 



A More Complex "Crossing Over" Process in which two interchanges have 



occurred. After Morgan. 



that the assortment of the genes is not as if they were so many 

 glass balls of different patterns and of two different colours, for the 

 genes are held in definite linkage-groups. 



