Genetic Algorithms: Genes values should sum up to one

Genetic Algorithms: Genes values should sum up to one

I want to implement a genetic algorithm (I'm not sure about the
language/framework yet, maybe Watchmaker) to optimize the mixing ratio of
some fluids.
Each mix consists of up to 5 ingredients a, b, c, d, e, which I would
model as genes with changing values. As the chromosome represents a mixing
ratio, there are (at least) two additional conditions:
(1) a + b + c + d + e = 1
(2) a, b, c, d, e >= 0
I'm still in the stage of planning my project, therefore I can give no
sample code, however I want to know if and how these conditions can be
implemented in a genetic algorithm with a framework like Watchmaker.
(Offtopic: the Watchmaker support page suggests to use "watchmaker" as a
tag, however, this tag doesn't exist yet)