DNA is organized into chromosomes. The organisms genome is the DNA in all their chromosomes.
During meiosis, the process that creates eggs and sperms, homologous chromosomes line up and recombine, meaning their sequences get mixed up together to form new chromosomes. One of each of these new chromosomes is independently assorted into daughter cells, so that each egg and sperm contain only one chromosome necessary (1n). When the sperm combines with the egg, the zygote (new organism) contains a copy of a homologous chromosome from each parent (2n).