Final answer:
A haploid cell has half of a full set of chromosomes, totaling 23 chromosomes in humans. These cells are gametes like sperm and egg, and they form a diploid zygote with 46 chromosomes upon fertilization.
Step-by-step explanation:
A cell with only 23 chromosomes is considered haploid, and these cells are the gametes, such as sperm and egg cells. They have half of a full set of chromosomes. When two haploid cells, one from each parent, combine during fertilization, they create a zygote. The zygote has a full compliment of chromosomes, which means it is diploid, having two sets of 23 chromosomes, or a total of 46. This process of halving the chromosome number during gamete formation is called meiosis, which ensures that genetic diversity is maintained, and upon fertilization, the offspring will have the correct number of chromosomes.