Chromosomal Theory of Inheritance

Mendel's own three laws described, with remarkable precision, the PATTERN by which hereditary factors are transmitted from parent to offspring, expressed purely as ratios of observable phenotypes — but Mendel himself, working entirely before the discovery of chromosomes' role in heredity, had no way of knowing what these abstract "factors" physically were, or exactly where within the cell they might be located. That crucial connection between Mendel's abstract genetic factors and the physically visible structures of the cell was made independently, around 1902, by the American cytologist Walter Sutton and the German embryologist Theodor Boveri, working on grasshopper and sea-urchin cells respectively.

Sutton and Boveri each noticed a striking, and on close inspection quite exact, parallel between the behaviour Mendel had deduced for his abstract hereditary factors and the behaviour that could actually be directly OBSERVED down a microscope for chromosomes during the process of meiosis. Just as Mendel's factors occur in an organism as a PAIR (one member inherited from each parent), so too do chromosomes occur as PAIRS of homologues in a diploid cell. Just as the two factors of a pair SEGREGATE cleanly from one another during the formation of gametes (Mendel's Law of Segregation), so too do the two homologous chromosomes of a pair visibly and physically separate from one another at anaphase of meiosis I, moving to opposite poles of the dividing cell. Just as DIFFERENT pairs of factors ASSORT INDEPENDENTLY of one another when a dihybrid cross is followed (Mendel's Law of Independent Assortment), so too do different homologous chromosome pairs orient themselves entirely independently of one another when they line up on the metaphase I plate. And just as fertilisation restores the PAIRED condition of hereditary factors in the resulting zygote, so too does the fusion of one haploid egg with one haploid sperm restore the diploid, paired condition of chromosomes in the fertilised zygote.

Building directly on this precise and repeated parallel, Sutton and Boveri jointly proposed what is now known as the Chromosomal Theory of Inheritance: that Mendel's hereditary factors — what we today call genes — are physically located ON chromosomes, and that it is specifically the observable mechanical behaviour of chromosomes during meiosis that supplies the underlying physical explanation for the patterns of segregation and independent assortment that Mendel had derived purely from counting breeding ratios, with no knowledge whatsoever of cell biology. In the vocabulary this theory made standard, a GENE is a specific, well-defined stretch of DNA located on a chromosome that codes for one particular hereditary character or product, whereas a CHROMOSOME itself is a much larger structure of DNA wound together with associated proteins, capable of carrying many hundreds or even several thousand individual genes strung out sequentially along its length — with each individual gene occupying one particular, fixed position along the chromosome, called that gene's locus. …