The 20s & 30s were the age of gold for classic genetics mostly based on the fruitfly Drosophila.
Cytogenetics is needed for the understanding of cancer from Pap smears (George Papanicolaou, 1883-1962) on to rathercomplicated situations, some involving viral transfections, and others citing telomere shortening and aneuploidy as the causes of cancer. Cancer is a disease of old age. Ageing includes shortening of the telomeres, possibly leading to genome instability, aneuploidy and cancer.

In 1987, Kary Mullis invented the polymerase chain reaction (PCR), using a thermostable enzyme from a thermophilic bacterium discovered by Thomas Brock in a geyser in Yellowstone National Park. After PCR, the genome can be analyzed without breeding experiments.

This brief sketch almost brings us up to date !