Dr. Gero was a scientist debatedly without equal, when it came to robotics. In biology, he did rather little. One may argue of Cell, but he had little involvement it its creation, limited to the robotics used to colect genetic information. In biology resided a far different scientist.
Proffessor Drachma was bewhildered by the performance of "King Piccollo" all those years ago at the martial arts tournament. He was only a child at the time, so his parents were worried over his 'amusement.' Regardless, he studied genetic engineering, and would become the foremost studier of the humanoid genome.
Since Chiikyuu was destroyed before Cell's completion, or rather Cell was, all his work had to be his own.
He started with comparative to the Namek's to produce a Namek-Human hybrid, which ran into significant problems as Nameks reproduce asexually when humans do sexually. From there, he saught every humanoid he could find, from Ice-jiin to... Guldo... and began creating hybrids.
Due to difference in environment or origin, they were immensely unstable. Out of fifteen, only two lived to sexual maturity, one of which died soon after. Thus, the only survivor was dubbed Tressen, roughly translated to third war, but few know why it was named that. Perhaps it related to 1/8 of its genome being saiya-jiin, the most prevalent of its non-human genes... Unfortuneately, this creature could not survive long outside the lab, despite relative strengths. This was accreddited to its "sheltered" life without exposure to pathogens.
Almost automously resolving this complication, Tressen soon gave birth, to Holly.