Biology CHROMOSOMAL THEORY OF INHERITANCE

### KEY TOPICS

star Lack of Recognition of Mendel’s works
star Verification of Chromosomal Theory of Inheritance.
star Discovery of Chromosomes:

### LACK OF RECOGNITION OF MENDEL’S WORKS

● 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.

### DISCOVERY OF CHROMOSOMES:

● 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".

### VERIFICATION OF 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".