Why would a geneticist study a yeast cell or a fruit fly or a mouse in order to understand human genes and human biology?

Due to common origin and similarity of genes.

Explanation:

Fruit fly and yeasts are common model organisms that are used by scientists to understand universal biochemical pathways.The universal biochemical pathways exist across different organisms because they share a common origin and have similar genes. The existence of similar genes helps the scientists to manipulate these model organisms such as yeast and fruit fly in the laboratory to understand human genes and human biology as it is ethically prohibited to perform experiments on humans, however, the same can be done on the mouse, fruit fly and yeast cell. Humans share 44 % of their genes with the fruit fly, 26% with yeast cell and 92% with a mouse.