It is clear that Gilgamesh is presented as not your average epic hero through his description of being two thirds divine and one third human. The text seems to reveal that this is not a result of a natural process of birth, but more of a result of conscious divine design, as the following quote seems to suggest:
Two-thirds of him is god, one-third of him is human.
The Great Goddess designed the model for his body,
she prepared his form... beautiful, handsomest of men,... perfect...
The use of the verb "designed" suggests that Gilgamesh's supernatural identity was something that resulted in his supernatural body and physical abilities, not only based on his description of "handsomest of men" and being "perfect" in physical form, but also clearly based on his supernatural strength and abilities. The mortal part seems to be centred on his consciousness and awareness, and this is something that becomes painfully apparent as the text progresses and the reader is forced to identify that although Gilgamesh may have divine strength, he is nothing more than a rather arrogant young man who has a lot to learn. Genetically therefore Gilgamesh's makeup is impossible, but it is a result of divine intervention and based more on characteristics rather than genetics.