● Mendel published his work on inheritance of characters in `color{Violet}"1865"` but for several reasons, it remained unrecognised till `color{Violet}"1900"`.

● Firstly, communication was `color{Violet}"not easy"` (as it is now) in those days and his work could `color{Violet}"not be widely publicised"`.

● Secondly, his concept of `color{Violet}"genes (or factors"`, in Mendel’s words) as stable and discrete units that `color{Violet}"controlled the expression"` of traits and, of the pair of alleles which `color{Violet}"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 `color{Violet}"mathematics"` to explain biological phenomena was totally `color{Violet}"new and unacceptable"` to many of the biologists of his time.

● Finally, though Mendel’s work suggested that `color{Violet}"factors"` (genes) were `color{Violet}"discrete units"`, he could not provide any `color{Violet}"physical proof"` for the existence of factors or say what they were made of.

● In 1900, three Scientists (`color{Violet}"de Vries"`, `color{Violet}"Correns"` and `color{Violet}"von Tschermak"`) independently rediscovered Mendel’s results on the inheritance of characters.

● Also, by this time due to `color{Violet}"advancements in microscopy"` that were taking place, scientists were able to carefully observe `color{Violet}"cell division"`.

● This led to the `color{Violet}"discovery of structures"` in the nucleus that appeared to double and divide just before each cell division.

● These were called `color{Violet}"chromosomes"` (colored bodies, as they were visualised by `color{Violet}"staining"`).

● By `color{Violet}"1902"`, the chromosome movement during meiosis had been worked out.



● `color{Violet}"Walter Sutton"` and `color{Violet}"Theodore Boveri"` noted that the behaviour of chromosomes was `color{Violet}"parallel"` to the behaviour of genes and used `color{Violet}"chromosome movement"` to explain Mendel’s laws .

● The important things to remember are that `color{Violet}"chromosomes"` as well as genes occur in pairs.

● The two alleles of a gene pair are located on `color{Violet}"homologous sites"` on homologous chromosomes.

● During `color{Violet}"Anaphase of meiosis I"`, the two chromosome pairs can align at the `color{Violet}"metaphase plate"` independently of each other.

● To understand this, compare the chromosomes of four different colour in the left and right columns.

● In the left column (`color{Violet}"Possibility I"`) orange and green is segregating together.

● But in the right hand column (`color{Violet}"Possibility II"`) the orange chromosome is segregating with the red chromosomes.

● `color{Violet}"Sutton and Boveri"` argued that the pairing and separation of a `color{Violet}"pair of chromosomes"` would lead to the segregation of a `color{Violet}"pair of factors"` they carried.

● Sutton united the knowledge of `color{Violet}"chromosomal segregation"` with Mendelian principles and called it the `color{Violet}"Chromosomal theory of inheritance"`.

● Following this `color{Violet}"synthesis of ideas"`, experimental verification of the chromosomal theory of inheritance by `color{Violet}"Thomas Hunt Morgan"` and his colleagues, led to discovering the basis for the variation that `color{Violet}"sexual reproduction"` produced.

● Morgan worked with the tiny `color{Violet}"fruit files"`, `color{Violet}"Drosophila melanogaster"`, which were found very suitable for such studies.



● They could be grown on `color{Violet}"simple synthetic medium""` in the laboratory.

● They complete their life cycle in about `color{Violet}"two weeks"`, and a single mating could produce a large number of progeny flies.

● Also, there was a clear `color{Violet}"differentiation of the sexes"` – the male and female flies are easily distinguisable.

● Also, it has many types of `color{Violet}"hereditary variations"` that can be seen with `color{Violet}"low power microscopes"`.