Let us make an in-depth study of the chromosomal theory of inheritance.
Mendel held that the traits were transmitted from generation to generation as discrete, stable, particulate units of heredity called factors, now termed as genes. He did not know the location of the factors (genes) in the cell because the existence of chromosomes in the nucleus, the role of nucleus in reproduction and the process of mitosis and meiosis were unknown at that time.
The discovery of these events threw considerable light on the mechanism of inheritance.
1. In 1875 Hertwig observed that during fertilization of sea urchin eggs, two nuclei, one from the sperm and one from the egg, fused together.
2. In 1875, Van Beneden discovered that the chromosome number doubled from 2 to 4 in threadworm on the fusion of sperm and egg nuclei.
3. In 1882, Flemming clarified the chromosomal events involved in mitosis, including the transfer of chromosomes from the parent cell to the daughter cells.
4. In 1887, Vas Beneden demonstrated meiotic (reduction) division.
5. In 1889, Boveri demonstrated the role of nucleus in the transmission of hereditary traits from the parents to the offspring.
On the rediscovery of Mendelian principles in 1900, the research on hereditary units started and soon led to the “chromosome theory of inheritance”.
W.S. Sutton and T. Boveri proposed the “chromosomal theory of inheritance” in 1902 and confirmed by T.H. Morgan in 1933. These two workers observed a closed similarity between the transmission of hereditary traits and behaviour of chromosomes while passing from one generation to the next through male and female gametes. According to this theory “chromosomes are the carriers of hereditary information, possess mendelian factors (genes), segregate and assort independently during transmission from one generation to the next”.
In other words, chromosomes are the physical basis of heredity because they have a special organization, individuality, functions and capable of self duplication. They play a vital role in heredity, mutation, variation and evolutionary development of the species.
Salient Points in Support of Chromosomal Theory of Inheritance:
1. Link between one generation and the next is through male and female gametes (i.e., sperm and egg). The two must carry all genetic/hereditary traits (characters).
2. Both the gametes contribute equally in the heredity of the offspring. The male gamete (sperm) provides only its nucleus to the female gamete (egg). Hence it follows that hereditary characters are controlled by nuclear materials, fusion of male and female gametes takes place during fertilization.
3. Nucleus possesses chromosomes. Hence, chromosomes must carry the hereditary traits.
4. Loss of a complete or part of the chromosome produces structural and functional deficiencies in the organism.
5. Like the hereditary traits, the chromosomes retain their number, structure and individuality throughout the life of an organism and from generation to generation. The two neither get lost or mixed up. They behave as units.
6. Both chromosomes as well genes (mendelian factors) occur in pairs in the somatic or diploid cells.
7. A gamete contains only one chromosome of a type and only one of the two alleles (of a trait).
8. The paired condition of both chromosomes as well as Mendelian factors is restored during fertilization.
9. Genetic homogeneity and heterogeneity, dominance and recessiveness can be predicted by chromosomal type and behaviour.
10. Homologous chromosomes undergo synapsis during meiosis and then separate or segregate independently into different cells which establishes the quantitative basis for segregation and independent assortment of hereditary factors.
11. In many organisms, sex of an individual is determined by specific chromosomes called sex chromosomes.
12. Biochemical studies reveal that hereditary units (genes) are composed of DNA in eukaryotes and RNA in some prokaryotes. The major amount of DNA is found in chromosomes proves beyond doubt that chromosomes are the carriers of hereditary units what we call genes.