The fusion of haploid sperm and egg nuclei during fertilization leads to the formation of a diploid zygote.

Explanation:
During sexual reproduction, the fusion of haploid gametes, which are the sperm and the egg, results in the formation of a diploid zygote. Let's understand this process in detail:

1. Haploid gametes:
- Sperm: Sperm cells are produced in the testes through a process called spermatogenesis. Each sperm cell is haploid, meaning it contains half the number of chromosomes as the parent cell (somatic cell). In humans, sperm cells contain 23 chromosomes.
- Egg: Eggs, also known as ova or oocytes, are produced in the ovaries through a process called oogenesis. Each egg cell is haploid and contains 23 chromosomes.

2. Fertilization:
- Fertilization is the process by which the sperm and egg fuse together to form a zygote. It typically occurs in the fallopian tubes of the female reproductive system.
- When a sperm cell reaches the egg, it undergoes a series of changes, including the release of enzymes that help it penetrate the protective layers of the egg.
- Once the sperm penetrates the egg, the nuclei of the sperm and egg fuse together, resulting in the formation of a diploid zygote.

3. Diploid zygote:
- The fusion of the sperm and egg nuclei combines the genetic material from both parents.
- The resulting zygote now contains a full set of chromosomes, with half of the chromosomes coming from the sperm and the other half from the egg.
- In humans, the diploid zygote contains 46 chromosomes, which is the normal diploid number.
- The diploid zygote then undergoes cell division through mitosis, leading to the development of an embryo.

Conclusion:
In summary, the fusion of haploid sperm and egg nuclei during fertilization results in the formation of a diploid zygote. This diploid zygote serves as the starting point for the development of an individual organism.