Underdominance is a somewhat obtuse term in genetics that refers to a situation where individuals that carry two different alleles of a gene (heterozygotes) are less fit than those that carry two identical alleles (homozygotes).  While not technically gene drive this phenomenon can have interesting consequences on the frequency of alleles that display underdominance and can, in theory, be used to introduce a genetic modification into a population and have it reach high frequencies, an outcome also achieved using gene drive.

The following video graphically illustrates what underdominance is and how it might be used to introduce a transgene into a population so that it reaches high frequencies.

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