Final answer:
A haploid spermatozoon fertilizes a haploid secondary oocyte to form a diploid zygote, the first cell of new offspring. This occurs as the secondary oocyte completes meiosis II upon fertilization, leading to the merging of genetic material from both gametes.
Step-by-step explanation:
When a haploid spermatozoon fertilizes a haploid secondary oocyte, the result is a zygote cell. The diploid secondary oocyte becomes a haploid ovum through the process of meiosis. A primary oocyte undergoes the first meiotic division, resulting in the formation of a secondary oocyte, which is arrested at the metaphase II stage. If fertilization occurs, the secondary oocyte completes meiosis II, ejects a second polar body, and becomes an ovum. Immediately upon fertilization by the sperm, the combined genetic material from the two haploid gametes results in the creation of the first diploid cell of the offspring, known as the zygote. This process involves the merging of the male and female pronuclei, which decondense, replicate their DNA, and intertwine their genetic material to form a single-celled organism with a full diploid set of chromosomes, prepared for mitotic cell division and subsequent development into a new human being.