Final answer:
Sex cells have half the number of chromosomes as other cells because they are haploid, while a zygote has a full set of chromosomes and is diploid. The zygote develops into an organism through mitosis.
Step-by-step explanation:
The number of chromosomes in sex cells and in a zygote differs due to the process of meiosis. Sex cells, such as sperm and egg, are haploid and contain half the number of chromosomes as other cells in the body. For humans, this means 23 chromosomes. On the other hand, a zygote is formed when the sperm and egg combine, resulting in a diploid cell with the full number of chromosomes, which is 46 chromosomes (23 pairs).
This difference is important because the zygote, with its full set of chromosomes, will serves as the blueprint for the development of the organism. The zygote undergoes mitosis, dividing into many cells that eventually form the body of the organism. The presence of two sets of chromosomes allows for genetic diversity and the combination of traits from both parents.