12.18:
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.
12.18:
Chromosomal Theory of Inheritance
In 1866, Gregor Mendel published the results of his pea plant breeding experiments, providing evidence for predictable patterns in the inheritance of physical characteristics. The significance of his findings was not immediately recognized. In fact, the existence of genes was unknown at the time. Mendel referred to hereditary units as “factors.”
The mechanisms underlying Mendel’s observations—the basis of his laws of segregation and independent assortment—remained elusive. In the late 1800s, advances in microscopy and staining techniques allowed scientists to visualize mitosis and meiosis for the first time.
In the early 1900s, Theodor Boveri, Walter Sutton, and others independently proposed that chromosomes may underlie Mendel’s laws—the chromosomal theory of inheritance. Researching sea urchins and grasshoppers, respectively, Boveri and Sutton noted striking similarities between chromosomes during meiosis and Mendel’s factors.
Like Mendel’s factors, chromosomes come in pairs. Reminiscent of Mendel’s law of segregation, these pairs become separated during meiosis such that every gamete (e.g., sperm or egg) receives one chromosome from each pair. Chromosome pairs are segregated independently of one another, corresponding to Mendel’s law of independent assortment.
The first concrete evidence for the chromosomal theory of inheritance came from one of its critics, Thomas Hunt Morgan. Morgan found that a mutation affecting the eye color of fruit flies was inherited differently by male and female flies, and demonstrated that this trait was determined by the X chromosome.
We now know that Mendel’s factors are DNA segments—called genes—at specific chromosomal locations. The independent assortment of genes on different chromosomes is a consequence of the random arrangement of chromosomes at the midline of a cell during metaphase I, which determines which genes segregate into the same daughter cells. Each homologous pair of chromosomes migrates independently of the others. The law of segregation corresponds to the movement of chromosomes during anaphase I which ensures, under normal conditions, that each gamete receives only one copy of each chromosome randomly distributed.