Chromosomal Theory of Inheritance

In the late 1800s, after Mendel had made his observations of inheritance in pea plants, improvements in microscopy allowed scientists to visualize sub-cellular structures in detail for the first time. After the processes of mitosis and meiosis were observed and described, it was proposed that chromosomes were Mendel's heritable particles.

The chromosomal theory of inheritance describes how the physical behavior of chromosomes accounts for Mendel's laws of segregation and independent assortment.

Organisms that reproduce sexually have two copies of each chromosome in their somatic, or non-reproductive, cells.

During meiosis, homologous chromosome pairs, those that carry the same genes, are split apart and segregated into the gametes so that each gamete receives one copy of every chromosome, just as Mendel described in his Law of Segregation.

Different, or nonhomologous, chromosome pairs are not linked to one another and sort into the gametes independently, as described in Mendel's Law of Independent Assortment.

Upon fertilization, the gametes combine their genetic material to produce an organism with two copies of each chromosome and thus two copies of each gene, one inherited from the mother and one from the father, just as Mendel predicted based on the patterns of inheritance he saw in pea plants.