MULTIPLE ALLELOMORPHS 203 
the new factor must act as an allelomorph to the first 
mutant, as well as to the “parent”? normal allelo- 
morph. 
Since these factors have the same location they 
must all give the same linkage values with other fac- 
tors. This has been shown to be true. For instance, 
the factor for white eye color of Drosophila is very 
closely linked to that for yellow body color. The 
“distance” between them is 1 unit, which means 
that crossing over takes place about once in a 
hundred times. Eosin eye color gives the same 
crossing over frequency with yellow. 
White eye color gives with miniature wings about 
33 per cent. crossing over. Eosin gives the same 
value with miniature. 
White gives 44 per cent. of crossing over with 
bar eye. Eosin has the same value. Similar rela- 
tions hold for all of the characters of the first group; 
they all have the same linkage values for eosin that 
they have for white. This example indicates that 
the conception of allelomorphs should not be limited 
to two different factors that occupy identical loci 
in homologous chromosomes, but that there may be 
three, as above, or even more different factors that 
stand in such a relation to each other. Since they 
lie in identical loci they are mutually exclusive, and 
therefore no more than two cah occur in the same 
animal at the same time. This is both demonstrated 
by the facts and postulated by the chromosomal 
mechanism. 
On a priori grounds also it is reasonable to suppose 
