Chromosomes are unrelated to Mendelian heredity and do not physically carry genes.

When Mendel’s work was rediscovered around 1900, his hereditary “factors” were mathematical entities inferred from breeding ratios. Chromosomes, meanwhile, were visible structures that paired and separated during cell division, but their connection to those factors was not yet established.

Walter Sutton and Theodor Boveri noticed that chromosome behavior during meiosis closely matched what Mendelian segregation required: homologous chromosomes occur in pairs, separate into gametes, and reunite at fertilization. The chromosome theory was initially a hypothesis rather than a settled identification. Thomas Hunt Morgan’s Drosophila experiments then gave it a physical foothold. The white-eye trait followed the X chromosome, and groups of linked traits recombined at frequencies that could be used to map their relative positions.

The convergence of cytology and breeding turned abstract genes into entities located on chromosomes. Modern molecular genetics later identified DNA as their principal material basis and refined what counts as a gene.