The genes of all organisms are arranged on the chromosomes . We can think of the genes as places on a long string. The plural of locus is loci . This means that if we want to find the gene for cystic fibrosis or Huntington's disease, we can find it on the same chromosome in the same place in all people. We can also construct maps of the chromosomes to show where these genes can be found, in much the same way that a highway map will show where cities and towns can be found along an interstate highway.
Most eukaryotes have at least some diploid cells in their life cycles.
We refer to the two copies of the chromosomes as homologous .