Chromosomal Theory

The chromosomal theory of inheritance is a theory of genetics that states that chromosomes are the transporter of genetic material from one generation to the next. Chromosomes are linear structures in which genes are located at specific sites. They have a thread-like structure containing proteins and nucleic acids. The theory was discovered in the early 1900s by two scientists, Walter Sutton and Theodore Bover. The theory was in alignment with the Mendelian law of heredity, which stated that genes, which he called factors, are the stable and discrete units that govern the transfer of traits from one generation to the next.

Chromosomes

In every cell nucleus, the DNA molecule is arranged into thread-like structures called chromosomes. These were discovered by Walther Flemming in the year 1882. In each chromosome, the DNA is strongly coiled several times around proteins that provide support to the structure. These proteins are called histones. When the cell is under normal conditions, i.e. not dividing, the chromosomes do not become visible even under a microscope. But during cell division, the DNA that forms the chromosomes becomes more tightly packed and becomes visible under a microscope. Most of the information collected about chromosomes was gathered while a cell was undergoing division under a microscope.

The chromosomes have a point called the centromere, by which they are divided into two sections, known as arms. The shorter arm of the chromosome is called the ‘p arm.’ The long one is called the ‘q arm.’ The characteristic shape of the chromosome is defined by its centromere point. It can be used to locate some specific genes.

Chromosomal Theory of Inheritance

Sutton and Boveri found that the detachment and pairing of a pair of chromosomes could lead to the separation of a pair of factors that they carried. Sutton unified the Mendelian principles with the chromosomal segregation theory and named it the chromosomal theory of inheritance. Following this theory, Thomas Hunt Morgan of Columbia University, along with his colleagues, tried to verify the chromosomal theory by experimentation. This led to the discovery of the basis of variation in the product of sexual reproduction. Morgan performed experiments on fruit flies called the Drosophila melanogaster. They could be grown simply in a laboratory on a synthetic medium. These flies have a life cycle of about two weeks, and a large number of flies could be produced by a single mating. The sexes in the flies could be distinguished easily. They also have many hereditary variations that can be observed with low-power microscopes. 

Chromosomal Theory of Inheritance: Observations

The Chromosomal Theory is based on Mendel’s laws. The observations of the theory are as follows:

• Homologous chromosome pairs move as discrete structures, independent of other chromosome pairs, during meiosis.

• The gametes are synthesized by each parent. These have only half of their chromosomal information.

• There is random sorting of chromosomes from every homologous pair into pre-gametes.

• The gametes of male and female, i.e. sperm and egg, have different sizes and structures. However, the number of chromosomes is the same in both the gametes, which shows that the genetic contributions are equal for each parent.

• During fertilization, the gamete chromosomes combine to produce offspring having the same number of chromosomes as its parents.

Gene Linkage

Morgan also studied sex-linked genes by dihybrid crosses in Drosophila. For instance, he did an experiment on yellow-coloured and white-eyed females with brown-coloured and red-eyed males. Their F1 offspring were intercrossed. He found that the two genes could not have independent segregation, and the F2 ratio deviated from the 9:3:3:1 ratio significantly. The genes were located on the X chromosome. Morgan saw that when the two genes in a dihybrid cross were situated on the same chromosome, the parental gene combination proportion was higher than the non-parental gene combination. He cited the physical association or linkage of the two genes as the reason and labelled this observation as the term linkage. He also observed that some of the genes were very tightly linked even when they were located on the same chromosomes, while the others were linked loosely and showed higher recombination. For example, he found that the genes white and yellow were very tightly linked and showed only 1.3 percent recombination, while the genes miniature wing and white had 37.2 percent recombination. 

Conclusion

In every cell nucleus, the DNA molecule is arranged into thread-like structures called chromosomes. The chromosomal theory of inheritance is a theory of genetics that states that chromosomes are the transporter of genetic material from one generation to the next. It was discovered in the early 1900s by two scientists named Walter Sutton and Theodore Bover and further explained by Thomas Hunt Morgan. Morgan also studied sex-linked genes by dihybrid crosses in Drosophila and saw that when the two genes in a dihybrid cross were situated on the same chromosome, the parental gene combination proportion was higher than the non-parental gene combination. This is termed linkage.