No. 601] MENDELIAN FACTOR DIFFERENCES 35 



performed in conjunction with a number, perhaps a very 

 great number or even all, of the other loci within the 

 system. With a change in this particular locus, how- 

 ever, comes a change in the normal course of events in 

 chlorophyll production, in that the rate at which the sys- 

 tem is able to produce chlorophyll has been altered. 

 Nevertheless, this change does not completely prevent the 

 system under favorable conditions from going on and 

 ultimately developing the same reaction end product 

 which would have developed in the normal unchanged 

 condition, but more rapidly. 



Among such factors as have a profound influence upon 

 the interrelations within the systems of which they are a 

 part are those which Morgan (1914) has called lethals. 

 Morgan's work with lethals is particularly suggestive 

 because he has been able to demonstrate that they, like 

 other normal Mendelian factors, occupy a definite locus in 

 the chromatin system and display the same perfectly defi- 

 nite and consistent behavior with reference to the other 

 loci of the system as do all other changed loci which do 

 not interfere with the normal development of the indi- 

 vidual. It is entirely possible that some of the lethals, 

 like the chlorophyll reduction locus which we have dis- 

 cussed above, may yield systems which occasionally per- 

 mit of the normal development of the individual, at least 

 certain peculiar sex ratios which have been obtained 

 might indicate that fact (Morgan, I.e.)-, but the important 

 result of these investigations with lethal factors lies in 

 that fact that certain kinds of clianges in some loci are 

 incompatible with normal functioning of the chromatin 

 system. It might in addition be noted that there seems to 

 be no particular reason why we should not include in the 

 same category with lethals, the type of chlorophyll reduc- 

 tion mentioned above and those other types in maize 

 which result from such profound factor changes that no 

 development is possible after the food supply of the endo- 

 sperm is exhausted. 



Now giving the above results their broader and more 

 general interpretation, it would appear that the factors 



