192 GENETICS 



of these contains certain materials in two doses (a pair of 

 genes), while the other contains but one dose (an unpaired 

 gene) . In consequence the two cells differ greatly in develop- 

 ment. In this way difference in sex is produced, with its 

 manifold attendant differences in structures and functions. 

 In many cases two genes of a certain kind produce a differ- 

 ent effect from a single one, though in other cases they do 

 not. That is, two centers of a certain kind of action may 

 give different results from a single center. This principle of 

 the effects of difference in balance is presumably of great 

 importance, though as yet not a very great deal is known as 

 to its operation. 



It appears possible that the operation of some or all of 

 the genes begins in early stages of development and con- 

 tinues throughout. It is known that some genes produce their 

 distinctive effects at the beginning of development, in the 

 egg itself. Thus in the silkworm a difference of one gene is 

 known to make a difference in the color of the egg. Two races 

 differ in egg color, one having slate-colored eggs, the other 

 brown eggs. Crossing the two races shows that the differ- 

 ence in color is due to a difference in a single gene, for the 

 descendants show "unit difference" inheritance. If, by cross- 

 ing, the special gene from the slate-colored race is brought 

 into the eggs of the brown race, in the next generation the 

 eggs are slate-colored instead of brown, so that the "slate" 

 gene is active in the formation of the egg. 



Other genes are known to produce their distinctive ef- 

 fects somewhat later, but still rather early in development. 

 Thus in mice, if one of the genes of a certain pair is modified 

 in a certain way, this causes the hair of the individual to be 

 yellow. If both the genes of this pair are so modified as to 

 produce the yellow color, the individual develops for but a 

 short time, then dies while still an embryo. It is clear that 

 these genes are active in the young embryo. Again, in plants, 

 certain genes are required for the production of chlorophyll. 



