Give the experimental verification of the chromosomal theory of inheritance.

Explanation

Mendel published his work on inheritance of characters in 1865 but for several reasons it remained unrecognised till 1900.
= Firstly communication was not easy in those days and his work could not be widely publicised.
= Secondly, his concept of genes (factors) as stable and discrete units that controlled the expression of traits and of the pair of alleles which did not .blend. with each other was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature.
= Thirdly, Mendel.s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.
= Finally, though Mendel.s work suggested that factors (gene) were discrete units he could not provide any physical proof for the existence of factors or say what they were made of.
= In 1900 three scientists (de Vries, Correns and Von Tschermak) independently rediscovered Mendel.s results on the inheritance of characters.
= Also by this time due to advancements in microscopy that were taking place, Scientists were able to carefully observe cell division.
= This led to the discovery of structures in the nucleus that appeared to double and divide just before each cell division. These were called chromosomes (coloured bodies, as they were visualised by staining).
= By 1902, the chromosome movement during meiosis had been worked out.
= Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement to explain Mendel.s laws.
= They united the knowledge of chromosomal segregation with Mendelian principles and called it chromosomal theory of inheritance.
= According to this theory : (i) All hereditary characters must be with sperms and egg cell as they provide bridge from one generation to the other. (ii) The hereditary factors must be carried by the nuclear material. (iii) Chromosomes are also found in pairs like the Mendelian alleles. (iv) The two alleles of gene pair are located on homolous sites on the homologous chromosomes. (v) The sperm and eggs have haploid sets of chromosomes which fuses to re-establish the diploid state. (vi) The genes are carried onto the chromosomes. (vii) Homologous chromosomes synapse during meiosis and get separated to pass into different cells.
= This is the basis for segregation and independent assortment.