RELATION OF GENES TO CHARACTERISTICS 191 



the body whose cells have lost one X-chromosome, the re- 

 cessive characters are manifested, while the remainder of 

 the body is dominant. In this way are produced mosaic or- 

 ganisms, showing diverse hereditary characteristics in differ- 

 ent parts of the body. Such mosaic diversities however ap- 

 pear to be of a very different type from the differentiation of 

 the body into tissues and organs. Whether any differentia- 

 tions in ordinary development are thus produced through 

 loss or modifications of certain genes must be considered as 

 yet uncertain, though it is entirely clear that most of them 

 are not so produced. 



But diversities between individuals, as we have seen, 

 commonly result from differences in the genes that they 

 contain. If two fertilized eggs begin with different sets of 

 genes, the processes of changing the cytoplasm, of differ- 

 entiating it into tissues and organs, differ in the two cases, 

 so that individuals with diverse characteristics are produced. 

 Very marked differences in characteristics may be produced 

 even when the two sets of genes differ In but a single gene 

 or pair of genes. In Drosophila, as before seen, if two 

 eggs differ in a single pair of genes in chromosome IV, one 

 of them produces individuals without eyes, while the other 

 has normal eyes. If two individuals differ in an entire chro- 

 mosome (X), one becomes a male, the other a female. 



Such differences are due to the diverse chemical and phys- 

 ical action of the different genes. They produce differences 

 among different cells, in the way set forth in earlier para- 

 graphs. Also, they may produce diverse chemicals (hor- 

 mones) which circulate through the body, in the way 

 described in Chapter III, thus causing changes In the devel- 

 opment of parts with which they come in contact. 



Different chemical and physiological effects, different de- 

 velopment, may be produced by difference in balance among 

 the genes, as shown in Chapter III. Two cells may contain 

 the same kinds of materials (genes) throughout, but one 



