836 TEXTBOOK OF ZOOLOGY 



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

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

 of genes and chromosomes and, especially in Drosophila, it has enabled 

 us to locate the positions of many of the genes on their respective 

 chromosomes. If two linked traits are separated every time crossing 

 over occurs, it is obvious that their genes lie at opposite ends of the 

 chromosome, but if they are separated only half the times, it is ap- 

 parent that half the length of the chromosome lies between them. 

 Therefore, by computing the percentages of such separations and 

 noting which traits are affected, it has been possible to make chromo- 

 some maps showing the positions of the various genes and the distances 

 separating them. The chromosome maps for Drosophila are quite com- 

 plete (Fig. 437). 



Mutations 



When chromosomes separate from each other at synapsis, aside from 

 breaking so that a piece of one will adhere to the correlative piece of 

 the other, various additional "chromosomal accidents" will occur 

 which change the organization of the chromosome. A piece of chro- 

 mosome, for example, might break off from the end of one chromosome 

 and adhere to the end of its homologue. Any disarrangement of the 

 genes of a chromosome results in structural or physiological changes 

 in the organism into which it goes. Such changes are inheritable and 

 are called mutations. Since there are places on the chromosomes where 

 these accidents occur more frequently than at other places, there is a 

 tendency for certain mutations to occur with fairly determinable reg- 

 ularity, say once in every hundred thousand births. Some mutations 

 are useful and are preserved to the species to such an extent that De 

 Vries believed they were the principal factor in bringing about evolu- 

 tion. Other mutations are a disadvantage to the species, and the or- 

 ganisms possessing them are eliminated in Nature's fierce struggle 

 for existence unless they are saved from that cruel fate by man's 

 interference. 



Human Heredity 



Man, very naturally, is interested most in the heredity of man. In 

 spite of this supreme interest, his knowledge of his own heredity is 

 much more limited than his knowledge of inheritance in any of a num- 

 ber of other organisms. There are two reasons for this : namely, he is 

 not free to experiment with his own kind; and, as would be expected. 



