248 



ESSENTIALS OF ZOOLOGY 



color vision. Of course, from the mating of such a person with a color- 

 blind man, all the sons and half the daughters will be color blind, but 

 a young woman who is used to having her father buy orange-colored 

 neckties, thinking they are blue, is not likely to marry a man similarly 



defective. 



Crossing Over 



Genetics might be defined as an exact science which has exceptions 

 to many of its rules. A previous paragraph describes the principle of 

 linkage which provided for the inheriting as groups of those traits 

 whose genes occur together on a chromosome. Certain gametogenetic 

 accidents bring about occasional exceptions to this principle. It will 



A 



C A 



B 



C A 



D D 



B D 



B 



Fig. 82. — Crossing- over occurs when chromosomes break apart after synapsis. 

 AB and CD are the original chromosomes while AD and CB are the chromosomes 

 resulting from the cross-over. 



be remembered (Fig. 60) that at synapsis the chromosomes pair 

 and the homologous chromosomes become loosely attached to each 

 other. At times, the pairing chromosomes will even twist around each 

 other. When they come to separate in the maturation division, instead 

 of making a complete separation they sometimes break off at some 

 point So that a part of each will be joined to a reciprocal part of the 

 other, as is shown in Fig. 82. Thus, as a result of crossing over, in- 

 stead of traits A and B, and C and D being associated in the offspring 

 as they were in the respective parents, A will be associated with D, 

 and C with B. 



Crossing Over, a Useful Tool. — The process of crossing over has 

 proved to be a valuable means of determining something of the nature 



