Chromosomal Theory of Inheritance

Gregor Mendel worked out the rules of inheritance in pea plants by 1865, but his paper sat unread until 1900, when Hugo de Vries, Carl Correns and Erich von Tschermak each rediscovered the same laws independently. Around the same time, improvements in light microscopy let cytologists watch dividing cells directly, and they kept noticing the same worm-shaped, stainable bodies condensing inside the nucleus every time a cell divided. These were named chromosomes, from the Greek for 'coloured body', because early stains made them visible. An organism carrying two complete sets of chromosomes, one contributed by each parent, is called diploid, and every chromosome is essentially one very long, continuously coiled DNA molecule along which genes sit at fixed addresses called loci. Putting these two independent discoveries together gave rise to the Chromosomal Theory of Inheritance: Mendel's abstract 'factors' are real physical objects with a specific …