250 GENETICS 



gether into rows. The different grades are heritable; cer- 

 tain stocks are regularly darker, others lighter. If darker 

 and lighter individuals are mated, the two conditions show 

 Mendelian inheritance, the darker condition being domi- 

 nant (figure 51) ; all the Fi generation are darker, while in 

 F2 there are three dark to one light. The difference be- 

 tween the two thus depends on a difference in one gene. 



But it can be shown also that the extent of color depends 

 on the environment. If the salamander is kept continuously 

 for a long time (a year or two) on a light background, the 

 yellow spots increase in size : they may run together into 

 stripes or into large yellow areas, as in figure 51, B, so that 

 the animals become much lighter. If on the other hand they 

 are kept for long periods on a dark background the yellow 

 areas decrease in size; the animals become on the whole 

 darker. The extent of the yellow color thus depends both 

 on the genes and on the environment; it can be altered by 

 appropriately changing either. 



In all these cases the following general principles apply: 



1. What are really inherited (passed on from parent 

 to offspring) are the genes; that is, certain materials that 

 in certain combinations and under certain conditions give 

 rise to certain definite characteristics. 



2. With the same original genes, different environmental 

 conditions may induce the production of diverse character- 

 istics. 



3. Also, with the same environmental conditions, dif- 

 ferent genes may induce the production of different char- 

 acteristics. 



4. The same difference in characteristics that is in one 

 case produced by diversity of genes is in other cases pro- 1 

 duced by diversity of environment. 



There can be little doubt that these general statements 

 apply to many characteristics in man. Some combinations 

 of human genes form an individual that is a much better 



