12 THE THEORY OF THE GENE 



or its size, or to the regularity of the distribution of its 

 facets. Other characters involve the body color; others 

 the shape of the wings, or the distribution of its veins; 

 others the spines and hairs that cover the body. 



A second group of about 120 linked characters includes 

 changes in all parts of the body. None of the effects are 

 identical with those of the first group. • 



A third group of about 130 characters also involves all 

 parts of the body. None of these characters are the same 

 as those of the other two groups. 



There is a small fourth group of only three characters : 

 one involves the size of the eyes, leading in extreme cases 

 to their total absence ; one involves the mode of carriage 

 of the wings; and the third relates to the reduction in 

 size of the hairs. 



The method of inheritance of linked characters is given 

 in the following example. A male Drosophila with four 

 linked characters (belonging to the second group), black 

 body color, purple eyes, vestigial wings, and a speck at 

 the base of the wings (Fig. 10), is crossed to a wild type 

 female with the corresponding normal characters, that 

 may be called gray body color, red eyes, long wings, and 

 absence of speck. The offspring are wild type. If one of 

 the sons^ is now crossed to a stock female having the four 

 recessive characters (black, purple, vestigial, speck), the 

 offspring are of two kinds only, half are like one grand- 

 parent with the four recessive characters, and the other 

 half are wild type like the other grandparent. 



Two sets of contrasted (or allelomorphic) linked 

 genes went into this cross. When the germ-cells in the 

 male hybrid matured, one of these sets of linked genes 

 went into half of the sperm-cells and the corresponding 

 allelomorphic set into the wild type half of the sperm- 



1 It is necessary to make this reservation as to the male Drosophila, be- 

 cause in the female these same characters are not completely linked. 



