The reason why we use fruit flies is that it is easier to keep, and it is easier to study instead of using monkeys, fruit flies only have 4 pairs of chromosomes and they are also pretty similar to us, they also can get genes like cancer.
It all started when Thomas Hunt Morgan a biologist in the 1900s, this time in history people did not know how genes pass down to generations so Morgan wanted to find out how.
During Morgan's research, he found a male fruit fly with white eyes instead of the usual red eyes, and this was only passed on to the male fruit flies and not the females.
He found out that the generic factor that determined eye color was the same one that determined gender. And Morgan was able to get a Noble prize for his findings.
One of Morgen's students named Herman muller also used fruit files and found that X-rays and other ionizing radiations can cause genetic mutations by breaking apart chromosomes, and that can cause genes to mutate, get deleted, and multiply. And that could be passed on to the offspring.
Scientists found how an embryo works, and what an embryo can do, this research was named "Notch", and with "Notch" they found out what cells are too to transition other types of cells during "Differentiation".
Notch also keeps intact cell multiplication, cell survival, and equilibrium, meaning that any mutations and abnormal signaling can cause developmental problems and cancers.
And the use of these fruit flies is far from over, they are being used to study wound healing, the effects of pharmaceuticals, how the brain works, and cancer as a whole, teaching us about how tumors spread, and how certain cancer cells resist medication.
And they also have to sleep, so we study them for sleep too, to find out why we have to sleep.
Also, they are used in space travel, so we use them to test how space travel affects the human body.
And how microgravity works, and we know how the heart gets weaker as the heart doesn't have to fight gravity.