Mendel's hereditary units, the variant alleles of genes, are located on chromosomes. In formulating the Law of Segregation, that individuals carry two hereditary units that are segregated in gamete formation, Mendel was describing the result of meiosis. It is really important to connect Mendel's experimental results back to the underlying biological phenomena, to be able, for example, to understand genetic linkage, describing how alleles that are on the same chromosome are more likely to be inherited together. The chromosomal theory of heredity underlies Mendel's results.