GENETICAL CHARACTERS 11-18 



PLUS AND MINUS MODIFIERS 



These genes (especially minus modifiers) are closely related 

 to suppressors and are so named because they increase or dimin- 

 ish the phenotypic expression of a given gene without themselves 

 having any effect on the wild-type character. Their presence is 

 only detected when the gene which they modify is present in homo- 

 zygous condition. They are abundant in Neurospora as modifiers 

 of fluffy and other nonconidial mutants. When a cross heterozygous 

 for fluffy is analyzed, the four fluffy cultures from one ascus nearly 

 always fall into two categories, revealing the presence of genes which 

 affect fluffy without affecting normal. If we look at the recessive 

 gene as usually involving the loss of some synthetic ability, then 

 plus modifiers decrease the synthetic ability still farther. A sup- 

 pressor transforms the recessive into normal and is therefore a 

 minus modifier which diminishes the character to zero expression 

 by increasing the synthetic capacity of the organism to normal. 

 Minus modifiers result in different degrees of diminution of the 

 character without producing a complete transformation to normal. 

 In this respect they resemble the Anaphragmic mutation of Lwoff 

 (1946). 



