Having two sets of chromosomes, one inherited from each parent, is key to the life cycles of all sexually reproducing organisms. Almost all human cells are diploid, that is they contain two homologous sets of chromosomes. The total number of chromosomes, 46 in humans, is referred to as the diploid number. The exceptions are egg and sperm cells, known as sex cells, or gametes.

Each gamete has a single set of chromosomes, one from each homologous pair. A cell with a single set of chromosomes is called a haploid cell. For humans, the haploid number is 23.These haploid cells are produced through a process of meiosis.

In the human life cycle, the nucleus of a haploid sperm cell from the father fuses with the nucleus of a haploid egg cell from the mother. The fusion of the nuclei along with the cytoplasm from the gametes is called fertilization. The resulting fertilized egg, called a zygote, is diploid has two homologous sets of chromosomes, one set from each parent. During the rest of the life cycle, the zygote develops into a sexually mature adult with trillions of cells produced by mitosis. In this way, fertilization restores the diploid chromosome number, and the zygote’s 46 chromosomes are passed on to all the other diploid body cell. An Easy way to remember: Haploid Half Diploid Double.