Penetrance?

[Paul G Johnson-1]

What is the general view of the term and phenomena of "penetrance?" This 
refers to differential expression of a phenotype under various environmental 
conditions or genetic backgrounds, although the genotype of a plant or line is 
the same. It seems to be a rather vague term.

My question arises because I may be seeing differing "penetrance" of 
flowering genes in Poa annua due to environment and genetic background.

PJ
john0130 at gold.tc.umn.edu