540 GENERAL CONSIDERATIONS 



(xx) will have no color-blind children, since the zygotes will be either xy 

 or XX. In the F2 generation, however, one-half the grandsons and one- 

 half the granddaughters are free from this defect and the other half of 

 the granddaughters carry the gene for color blindness as a recessive. A 

 normal father with a color-blind mother will have only color-blind sons 

 while the daughters will be normal. Also one-half the grandsons Lnd 

 one-half the granddaughters will be color-blind. There are many such 

 sex-linked characters. These should not be confused with other charac- 

 ters 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. 



612. 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 the genes are carried in the same chromosome. In partic- 

 ular 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 particular chromosomes but also at particular points in those 

 chromosomes. 



613. 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. 



