but a
negative effect on the second character 
. 
on character
The genotypic values are then determined by adding all allelic effects of
the alleles present in the genotype:
. This is a highly symmetric case where two
alleles 
and 
both have the same effect d on
character 1 but an opposite effect of the same magnitude on
character 2 (d or -d). In the corridor model the allelic
effects point with one component into the direction of character
1 and thereby `uphill' to higher fitness values. Simultaneously
the pleiotropic effects on character 2 lower the fitness because
they move the genotypic values towards the flanks of the ridge.
Therefore the pleiotropic effects are on the average deleterious.
However, due to the additivity of the allelic effects, the
deleterious pleiotropic effects can compensate each other at the
level of the genotype.