CAUSE OF GERMINAL VARIATIONS 



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due to some germinal variation. They may be dominant or 

 recessive in character. They are of tremendous economic impor- 

 tance to man since valuable and desirable mutants can be used as a 

 starting point for a new species of plants or animals. De Vries was 

 the first man to advance the theory that many of our present-day 

 plants and animals possibly originated as mutants. 



Causes of germinal variations. Our ignorance concerning the 

 causes of germinal variations is profound. We do know that slight 

 variations are exceedingly common ; no two living things, even of the 

 same species, are exactly alike. Decided variations such as muta- 

 tions are more rare, but when we become more observant we find 

 them to be more frequent than, at first, believed. The following 

 are some of the more common explanations of causes of mutations. 



(a) Changes in ckromosome number. Scientists found that the 

 original primrose had 14 chromosomes, but that the 7 mutants 

 had respectively 15, 16, 20, 22, 24, 27, and 28 chromosomes. 

 Other observers have also found at times a different number of 

 chromosomes in other mutants. How or why the number of chro- 

 mosomes changes is not yet definitely known. 



AB 



a B 



Chromosomes A and B sometimes bend around each other. Before they separate, part of 

 B becomes joined to A as in C and part of A becomes joined to B as in D. This is called 

 crossing over. Some of the factors originally in one chromosome are now in the other. 



(h) Change in the character of the gene itself. Morgan has found 

 that most of the fruit fly (Drosophila) mutants that he has studied 



