Pairs of homologous chromosomes are present in the genome of diploid organisms. For example, in the genetic makeup of humans, there are 23 homologous pairs of chromosomes, making a total of 46. Two chromosomes are a homologous pair when they are very similar, sharing most of their DNA (although differences in the precise sequence of nitrogenous bases will be present when the members of the pair carry different alleles for some gene; also heteromorphic chromosomes, such as the X and Y sex-determining chromosomes in humans, can still be mostly homologous). This similarity causes the members of the pair to be attracted to each other during meiosis. The 4 haploid cells produced in meioisis will each have one member of a homologous chromosome pair.