In a manner involving addition; by combining things together cumulatively.
From 'additive' (relating to addition) plus the adverbial suffix '-ly'. This adverb form allows speakers to describe how something happens through additive processes.
In genetics and chemistry, 'additively' describes how traits or effects combine—if a child gets brown eyes from both parents 'additively,' both parents' contributions matter together, not one overriding the other.
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