Historical Development of Chromosome Theory

Wilhelm Roux

Wilhelm Roux (1850 – 1924):

• Wilhelm Roux was a German zoologist and pioneer of experimental embryology.
• Wilhelm Roux (1883) postulated that the chromosomes of a cell are responsible for transferring heredity.

Montgomery:

• Montgomery (1901) was the first to suggest the occurrence of distinct pairs of chromosomes.
• He also concluded that maternal chromosomes pair with paternal chromosomes only during meiosis.

Theodor Boveri

Theodor Heinrich Boveri (1862 - 1915):

• Theodor Boveri (1902) supported the idea that chromosomes contain genetic determiners.
• He was largely responsible for developing the chromosomal theory of inheritance.

Walter Sutton

Walter Stanborough Sutton (1877 – 1916):

• Walter Stanborough Sutton was a young American student who independently recognized parallelism (similarity) between the behaviour of chromosomes and Mendelian factors during gamete formation.

Boveri-Sutton chromosomal theory of inheritance (1903):

• In 1902, Walter Sutton and Theodore Boveri observed that behaviour of chromosomes was parallel to that of genes. They also explained Mendel's laws on the basis of chromosomal movements.
• On the basis of the appearance of chromosomes during meiosis, it was evident that:
  a) Chromosomes occur in pairs, similar to genes.
  b) Two alleles of a gene pair are located on the homologous sites on homologous chromosomes.
• Genes are located on the homologous pair of chromosomes. Thus, during gamete formation, homologous chromosomes pair and segregate independently at meiosis and go to different gametes.
• During fertilization, when gametes belonging to two different - homologous pair fuses, a new pair of alleles is formed, which ultimately results into a new combination.
• This helps to explain the Mendelian segregation and independent assortment.
• Sutton gathered the knowledge of chromosomal segregation with Mendelian principles and called it as 'Chromosomal theory of inheritance'.

Thomas Hunt Morgan

Support for chromosomal theory of inheritance: 

• This theory was widely discussed and controversies by scientists around the world. However, this debate has been finally cleared by the works of Thomas Hunt Morgan (1910) on the fruit fly Drosophila melanogaster (2n = 8).
• This fruit fly completed their life cycle within two weeks.
• The alleles for red or white eye colour are present on the X chromosome but there is no counterpart for this gene on the Y chromosome. Thus, females have two alleles for this gene, whereas males have only one.
  

  Structure of somatic and sex chromosomes in Drosophila and sex linkage

• The genetic results were completely based on the meiotic behaviour of the X and Y chromosomes. Similarly, the genes for the yellow body colour and miniature wings are also carried on the X chromosome.
• This study strongly supports the idea that genes are located on chromosomes.
• The linked genes connected together on sex chromosome is called sex linkage.

• The reasons for selecting Drosophila melanogaster were as follows:

1. These fruit flies can be easily grown on a simple synthetic medium in the laboratory.
2. Drosophila has a short life span of 2 weeks.
3. A single mating gives rise to a large number of progeny flies.
4. Male and female flies are easily distinguishable.
5. It possesses many types of hereditary variations that can be (Male) (Female) observed easily even under a low power microscope.

• Thomas Hunt Morgan (1933) received Nobel Prize in Physiology or Medicine for his discoveries concerning the role played by chromosomes in heredity.