184 INBKEEDING AND OUTBREEDING 



ternal in their effect and of more fundamental importance. 

 These more fundamental factors are those concerned di- 

 rectly with metabolism and cell division. As to their na- 

 ture and the way in which they are inherited, we, as yet, 

 know little, but there is reason for supposing they are 

 Mendelian in mode of inheritance and operate in a way 

 to enable the hybrid progeny to attain a greater develop- 

 ment than either parent. 



For a definite detailed case showing exactly how dom- 

 inance and linkage thus work together, we must look to the 

 work with Drosophila melanogaster, as this is the only 

 material which at the present time has been sufficiently 

 well analyzed for our purpose. Bridges and Sturtevant 

 have discovered, isolated, and determined the linkage re- 

 lations of nearly one hundred factors distributed through- 

 out the four chromosomes of this little fly, in the great 

 majority of which the recessive condition is unfavorable. 

 Through this indefatigable work there is an enormous 

 amount of data from which to choose ; but in order to make 

 our illustration comparatively easy to follow, let us con- 

 sider only four characters which are linked together in the 

 second chromosome. The factors which have the princi- 

 pal effect on these characters may be given the name 

 of the character. They are long legs (D) dominant to 

 "dachs" legs (d)< gray body (B) dominant to black body 

 (&), red eye (P) dominant to purple eye (p), and normal 

 wings (V) dominant to vestigial wings (v). In gamete 

 formation in the female there are breaks with a frequency 

 of 10 per cent, in the linkage between d and 5, of 6 per 

 cent, between b and p, and of 13 per cent, between p and v, 

 disregarding some disturbing conditions which need not 

 concern us here. In the male there are no linkage breaks. 



