Assertion is true. Fruit flies (Drosophila) exhibit clear differentiation of sexes. They have a dimorphic sex determination system, with males having XY chromosomes and females having XX chromosomes. The sex chromosomes determine the sex of the offspring. This clear differentiation of sexes is evident in their sexual dimorphism, with observable differences in physical characteristics between males and females.

Reason is also true. Drosophila is a widely used model organism in genetics research, primarily due to its short generation time, large number of offspring, and ease of maintenance in the laboratory. Many types of hereditary variations, including mutations and traits governed by specific genes, can be observed in Drosophila using low-power microscopes. 

Assertion and reason are correct and reason does not correctly explain the assertion.