The fruit fly has 8 chromosomes (2n) in each cell. During interphase of Mitosis if the number of chromosomes at G₁ phase is 8, what would be the number of chromosomes after S phase?

The fruit fly is diploid with a chromosome complement of $2n\; =\; 8$. This means that each somatic cell contains eight distinct chromosomes.

During the G₁ phase of interphase, each chromosome exists as a single chromatid, so the cell has eight chromosomes.

When the cell enters the S (synthesis) phase, DNA replication occurs. Each chromosome’s DNA is duplicated, producing two identical sister chromatids that remain attached at their centromere.

Although the amount of DNA has doubled, the number of chromosomes does not change because a chromosome is defined by its centromere, not by the number of chromatids. Therefore, after S phase the cell still contains eight chromosomes, each now composed of two sister chromatids.

Hence, the number of chromosomes after S phase is $8$.