When gametes combine to form a zygote, the chromosome number is now referred to as diploid.

Here's why:

* Gametes (sperm and egg) are haploid, meaning they have half the number of chromosomes as a normal body cell.

* When a sperm and egg fuse, their chromosomes combine, resulting in a zygote with the full complement of chromosomes.

* This full complement is known as diploid.

Therefore, the zygote has the same number of chromosomes as the parent organisms, restoring the diploid state.