Re: Is the effect of sum of components same as the sum of effect of components?

You are absolutely right that genes and their protein products interact,
both with each other and with the environment, to produce a particular
phenotype.  Plant breeders used to talk about finding a "happy home" for a
particular gene or trait in a plant, meaning that it wasn't enough to breed
one specific allele connected to a desirable trait into a commercial
variety.  You then have to breed for a whole genotype in which that allele
can be exploited fully.  There are, also, many phenotypic traits that are
produced by large numbers of genes working together – human intelligence,
however you wish to define it, is undoubtedly one such trait, for instance.
 I doubt seriously that anyone will ever find a "gene for high IQ" for that
reason.  

That said, one of the surprising findings over the past twenty years or so
has been that a number of pretty complex phenotypic traits are profoundly
influenced by a single gene.  Just this month, for instance, a study was
published in the journal Science (Sage et al., 2005) in which researchers
were able to induce new inner ear "hair cells" to grow by knocking out
expression of a single gene.  This restored hearing in mice whose hair
cells had been destroyed experimentally, and has some promise for the
treatment of hearing loss in humans.  Now it's true that the mice could not
grow the hair cells without the presence of lots of other genes, but the
activity of just one gene made the difference between growing them only
once, during embryonic development, and being able to re-grow them as adults.  

So it looks as if we can't assume that complex traits will always be
controlled by complex gene interactions.  In some important cases (some
more examples: cystic fibrosis, hemophilia, sickle-cell anemia, and the
inherited immune deficiency that afflicted the "boy in the bubble" are all
single-gene defects), altering a single gene can produce major effects. 
Actually doing the research is probably the only way we'll find out which
conditions fall into this category. 

Cyrille Sage, Mingqian Huang, Kambiz Karimi, Gabriel Gutierrez, Melissa A.
Vollrath, Duan-Sun Zhang, Jaime García-Añoveros, Philip W. Hinds, Jeffrey
T. Corwin, David P. Corey, Zheng-Yi Chen.  2005.  Proliferation of
Functional Hair Cells in Vivo in the Absence of the Retinoblastoma Protein.
  Science 307: 1114-1118.