Final answer:
When two haploid gametes, such as a sperm and an oocyte, combine, they form a diploid zygote. This is the first step in the development of a new multicellular organism.
Step-by-step explanation:
When two haploid gametes join during fertilization, they form a zygote. This zygote is diploid, meaning it has two complete sets of chromosomes, one from each parent. The process begins with the formation of a haploid secondary oocyte, which can only complete meiosis II and become an ovum if a sperm cell penetrates it. Upon fertilization by the sperm, the ovum rapidly transitions from being a haploid cell to becoming the first diploid cell of the offspring. The zygote then undergoes numerous rounds of mitosis, leading to the development of a multicellular organism.