比重 is usually used in chemistry and physics to describe the relative density or specific gravity of a substance or solution. However, in this sentence it simply sounds like the "relative weight", or in other words, the relative importance of two sets of factors.

This is how I would translate the sentence:
結腸・直腸癌の発生には、遺伝的因子よりも環境的因子の比重が大きく関わっていると考えら れている。

It appears the relative weight of environmental versus genetic factors plays a more significant role in the formation of colon and rectal cancer.

I am somewhat guessing about 比重, but it makes sense to me...

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Note added at 1 hr (2007-02-20 06:28:51 GMT)
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Youzoren's translation that starts with "it is generally believed..." is a better translation of と考えられている than my "It appears..."