Sexual reproduction at the molecular level is a very complicated sequents of events. Every gene is found on a specific chromosome. Ther are 46 chromosomes in humans, and each chromosome has a pair, so that there are 23 pairs of chromosomes (there are two copies of each gene). Sex cells are haploids, they only have one copy of each chromosome compared to all the other diploid cells in our bodies. Before the haploid cells are differentiated during meiosis, the chromosomes undergo a process called homologous recombination, where chromosomes of the same type exchange genes with each other. When the two haploid cells (one from the mother, one from the father) combine a zygote is created with 46 chromosomes, 23 chromosomes from each parent. Every gene has a pair and there are two genotypes: homozygous, where both genenes are the same, and heterozygous where the genes are similar, but different. How the gene are expressed in the body is known as the phenotype. Many different phenotypes are possible depending on genes in question.