He was renowned as the first person to recognize the normal number of human chromosomes on 22 December 1955 at the Institute of Genetics of the University of Lund in Sweden, where he was a visiting scientist.

Tjio was born to Indonesian parents of Chinese origin in Pekalongan, Java, then part of the Dutch East Indies and later known as Indonesia.

Tjio was educated in Dutch colonial schools, trained in agronomy in college, and did research on potato breeding.

From 1948 to 1959 he did plant chromosome research in Zaragoza in Spain and spent his summers in Sweden working with Professor Albert Levan in Lund.

[3] In 1953, a lab mistake involving mixing HeLa cells with the wrong liquid led Tjio and Levan to develop better techniques for staining and counting chromosomes.