Biology

POLYGENIC INHERITANCE :

Inheritance of characters in which one character is controlled by many genes and intensity of character depends upon the number of dominant allele.

Polygenic inheritance first described by Nilsson - Ehle in kernal colour of wheat.

Nilsson - Ehle said that kernal colour of wheat is regulated by two pairs of gene.

Example-2.


Colour of the skin in Human.
The inheritance of colour of skin in human studied by Devenport.

Human skin colour is regulated by 3 gene pairs.

When a Negro Black (AA BB CC) phenotype is crossed with white (aa bb cc) phenotype, intermediate pheno-
type produced in `F_1` generation.

CHROMOSOMAL THEORY OF INHERITANCE

This theory was proposed by Walter Sutton and Theodor Boveri (1902).

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 (colored 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. Recall that you havestudied the behaviour of chromosomes during mitosis (equationpl division) and during meiosis (reduction division).The important things to remember are that chromosomes as ~ell as genes occur in pairs. The two alleles of a gene pair are located on homologous sites on homologous chmmosomes.

During metaphase of meiosis I, the two chromosome pairs can align at the metaphase plate independentaly of each other. To understand this, compare the chromosomes of four different colour in the left and right columns. In the left column (Possibility I) orange and green is segregating together. But in the right hand column {Possibility II) the orange chromosome is segregating with the red chromosomes.

Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of nheritance.

Following this synthesis of ideas, experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and
his colleagues, led to discovering the basis for the variation that sexual reproduction produced. Morgan worked with the tiny fruit files,
Drosophila melanogaster, which were found very suitable for such studies. They could be grown on simple synthetic medium in
the laboratory. They complete their life cycle in about two weeks. and a single mating could produce a large number of progeny tlies.
Also, there was a clear differentiation of the sexes the male and female flies are easily distinguisable. Also, it has many types of hereditary variations that can be seen with low power microscopes.

 
SiteLock