[R] T tests on multiple groups

[Ed Siefker]

I have a data set with observations on groups with multiple variables.
Let's call them GENO and AGE.  I have control and test genotypes
and two different ages.  It is only meaningful to compare control and
test within the same age.

I'd like to get the p value for each group compared back to control
of the appropriate age.  T-test requires that the grouping factor has
exactly two levels.   How can I do this efficiently?

I was hoping something like ttest(OBS ~ GENO * AGE, mydata) would work.
Is there something I can do with tapply() or aggregate() to do this?
I'd like to end up with a table that looks like this:

GENO    Age    OBS    p.val
control    10    1.1    1
control    10    0.9    1
control    20    2.1    1
control    20    1.9    1
A    10    11    0.01224066
A    10    9    0.01224066
A    20    21    0.003102783
A    20    19    0.003102783
B    10    4    0.057714305
B    10    6    0.057714305
B    20    14    0.005923285
B    20    16    0.005923285
AB    10    1    0.698488655
AB    10    1.1    0.698488655
AB    20    2    0.552786405
AB    20    2.2    0.552786405