[Biology Class Notes] on Chromosomal Theory of Inheritance Pdf

Where do genes find themselves in a cell? Odds are, you have heard the punchline before: genes lie on chromosomes. You may have even heard the second punchline, the one that is ushered in the modern genetic age: genes are DNA stretches specifying proteins.

 

Chromosomes are a thread-like structure of nucleic acids and proteins located within the nucleus of living cells and are primarily involved in gene-shaped genetic information. The Swiss botanist Carl Wilhelm von Nageli discovered chromosomes and was the first person to research the cell divisions.

 

The Chromosomal Theory of Inheritance

It is the fundamental genetic theory that recognizes chromosomes as the bearers of genetic material. Theodor Boveri and Walter Sutton are the two scientists who have been credited with developing the Inheritance Chromosomal Theory. In the early 1900s, Boveri and Sutton gave the chromosomal theory of inheritance. It is the fundamental genetic theory. According to this theory, genes are heredity units, and they are found in the chromosomes. Inheritance chromosomal theory came into being long after Mendelian genetics. Society wasn’t acceptable to such drastic changes in their scientific ideas during Mendel’s experimentation. They did not believe there were such distinct variables as genes that would Separate without combining, as this did not help their notion of continuous evolutionary shifts. 

 

In addition, the means of communication at that time were weak, as a result of which the information could not be transmitted to the masses. Mendel’s scientific approach to proving biological rules has always been inappropriate. Scientists Vries, Correns, and Tschermak found chromosomes that remained within the nucleus as time passed. Sutton and Boveri discovered when the cells were divided they observed the behaviour of the chromosomes. This task became simpler with the advances in the microscope. Thus, they proved the laws of Mendel with the aid of chromosomal motion. They showed chromosome segregation during the cell division Anaphase. The idea of chromosomal segregation combined with the Mendelian principles has given rise to Inheritance’s Chromosomal Theory. The research was carried out further and was proved by T.H. Morgan, who used Drosophila melanogaster to demonstrate how it induced differences in sexual reproduction.

 

Chromosomal Theory – Linkage and Genetic Recombination

Morgan found that two genes did not segregate as per Mendel’s law when crossing a range of characteristics. If two genes were present on the same chromosome, the probability of obtaining a parental combination in the next generation was much higher compared with the non-parental combination. This physical gene association was termed a connection. In a dihybrid cross, the word genetic recombination identifies the combinations of non-parental genes. Once connected genes were discovered, the frequency of connected genes also influenced trait appearance in the next generation. A Morgan’s student, Sturtevant, discovered the location of the related genes on a chromosome by measuring their genetic recombination frequency using the gene mapping technique. This method of producing a map of ties was commonly used during the Human Genome Project.

 

Observations of Chromosomal Theory of Inheritance

The inheritance chromosomal theory supports Mendel’s laws. The observations of this theory are listed below:

• The pairs of homologous chromosomes migrate as discrete structures independent of other chromosome pairs during the process of cell division-meiosis.;

• Chromosomes of each homologous pair are randomly distributed into pre-gametes.

• Gametes are synthesized by each parent and contain just half of their chromosomal complement.

• While the size and shape of the male and female gametes (sperm and egg) vary, they have the same number of chromosomes, indicating similar genetic contributions for each parent.

• During fertilization, gametic chromosomes combine to produce offspring with the same number of chromosomes as their parents.

Key Points

1. Boveri and Sutton’s inheritance chromosome theory states that genes are found at specific chromosome locations, and that chromosome behaviour during meiosis may explain Mendel’s inheritance laws. Thomas Hunt Morgan, who studied fruit flies, provided the first strong confirmation of the chromosome theory.

2. Morgan discovered a mutation affecting the colour of the fly-eye. He noted male and female flies inherited the mutation differently.

3. Following the pattern of inheritance, Morgan assumed that the eye colour gene must be located on the X chromosome.

What is Mutation?

The mutation is defined as the change or the permanent alteration of the nucleotide sequence of the genome of an organism. Our DNA can undergo changes in its basic sequences A, C, G and T. This results in changes in proteins that are DNA-synthesized. Generally, the cells can identify and restore any damage that could be done by mutation until it is permanent. 

 

The Causes of Mutation 

The mutation results in genetic differences between the species. Few mutations with positive effects are transferred to successive generations. For example, sickle cell anaemia is the result of a mutation in the gene that codes for the synthesis of the protein haemoglobin. The red blood cells take the form of a sickle. Within the African population, however, this mutation also protects against malaria. 

 

Cancer is the most common mutation-related disease in a number of growth-control cells.