INHERITANCE IN ORGANISMS 569 



daughters will be color-blind. There are many such sex-linked char- 

 acters. These should not be confused with other characters which 

 distinguish one sex from the other, since the corresponding genes are 

 carried in autosomes and the characters are developed under the influence 

 of hormones produced in either the ovary or the testis. 



613. Linkage and Crossing Over. — It is evident that many charac- 

 ters not associated with sex may be linked in inheritance because of the 

 fact that genes are carried in the same chromosome (Fig. 391). In certain 

 cases, however, it is found that such linkage is broken. This can be 

 explained only by assuming that when two chromosomes unite in synapsis 

 there is fusion at certain points and that when they separate portions of 

 the two are exchanged. This is called crossing over. The existence of 

 such linked characters and of crossing over has been seized upon as a 

 means of determining the position of genes in the chromosomes. In 

 the fruit fly, Drosophila, where a number of such characters occur, it 

 has been found that by comparing cases of variation in linkage the chances 

 of such crossing over vary. It may be assumed that when genes are far 

 apart the chances are greater than when they are close together. As a 

 result of thousands of experiments by Morgan and his students it has 

 been found possible in the case of the fruit fly to locate the genes con- 

 cerned in the production of a large number of characters not only in par- 

 ticular chromosomes but also at particular points in those chromosomes. 



614. Eugenics. — The application of the principles of genetics to the 

 production of human offspring with the aim of developing a superior 

 race has been strongly urged. This field of investigation and social 

 endeavor is known as eugenics. There are many individuals and organ- 

 izations seeking to develop an intelligent eugenic program, to secure 

 its adoption, and through it to improve the human race. The adoption 

 of such a program, however, can be brought about only as a result of 

 long-continued agitation and the gradual education of the public. 



