A gamete is a haploid sex cell of an organism that reproduces sexually.
When the haploid nuclei of two gametes fuse at fertilization, the resulting zygote or fertilized egg contains the diploid number of chromosomes found in body cells of that particular organism. From that point on, the genetic blueprint of that organism has been determined and all cells that are produced from that fertilized egg cell will contain the diploid number of chromosomes. Only gametes contain half the number of chromosomes and are produced by a special reduction division known as meiosis to be used during sexual reproduction.
Different species contain different chromosome numbers. For example, Homo sapiens which are modern humans contain 46 which is the diploid number of chromosomes in their body cells and 23 which is the haploid number of chromosomes in their gametes. Gametes contain half the amount of chromosomes found in that species.
Drosophila melanogaster is the fruit fly commonly used as a subject of study in many genetics experiments. The diploid amount of chromosomes in their body cells is 8 and the haploid number in their gametes is 4.