gene
Today, we known that characteristics of organisms are controlled by genes on chromosomes see Figure below . The position of a gene on a chromosome is called its locus . In sexually reproducing organisms, each individual has two copies of the same gene, as there are two versions of the same chromosome homologous chromosomes . One copy comes from each parent. The gene for a characteristic may have different versions, but the different versions are always at the same locus. The different versions are called alleles . For example, in pea plants, there is a purple-flower allele B and a white-flower allele b . Different alleles account for much of the variation in the characteristics of organisms.