[R] Adding same items together in data.frame

[wong, honkit (Stephen)]

Dear All,
I am new to R. I have a 2 column data frame with more than ten thousand
rows. Something like below. I want to add up all duplicated items, e.g. the
three "aa" add up together to get a single value gene=a, value=74. How can I
do that?? Thanks for help !
gene value
aa	 20
bb 	10
cc	 9
aa 	30
aa 	24
dd	 100
ee 	55
.... ...
Millions thanks.
Best Regards,
hon

WONG, Hon-Kit (Stephen)
Cleary Lab, Dept of Pathology
Stanford University