Organisms that undergo sexual reproduction have one set of chromosomes contributed by each parent.In humans, before fertilisation the egg cell has half the number of chromosomes and is said to be haploid. The sperm cell is also haploid and has the same number and type of chromosomes as the egg cell with the same genes in the same loci. Meiosis is a type of cell division that gives rise to haploid gametes.During fertilisation the egg cell nucleus fuses with the haploid sperm cell nucleus to give a zygote. The zygote has both sets of chromosomes so is said to be diploid.A pair of chromosomes with the same genes in the same loci, one originating from the female parent and the other originating from the male parent, is called homologous chromosomes.Alleles are different forms of the same gene. Homologous chromosomes have the same genes at the same loci, but the alleles may be different.