Final answer:
The union of a human egg and sperm cell forms a zygote, which is diploid and has 46 chromosomes.
Step-by-step explanation:
The cell produced by the union of a human egg and a human sperm cell is called a zygote. This cell is diploid, meaning it has two sets of chromosomes, one from each parent. Therefore, a human zygote possesses 46 chromosomes in total,
which is comprised of 23 pairs of chromosomes. Upon fertilization, sperm and egg gametes, which are haploid cells containing 23 chromosomes each, merge to form the zygote. The zygote then goes through several rounds of mitosis to develop into a multicellular human organism.