The science of genetics is built upon the foundation of understanding how traits are passed from one generation to the next. Gregor Mendel, the "Father of Genetics," explained the principles of inheritance through his experiments on pea plants in 1866. However, during Mendel’s time, the physical material responsible for heredity was unknown.
Later cytological studies revealed that chromosomes, thread-like structures in the nucleus, play a crucial role in inheritance. The chromosomal theory of inheritance, proposed by Walter Sutton (1902) and Theodor Boveri (1902–1903), connected Mendel’s laws with the behavior of chromosomes during meiosis. This theory established that genes are located on chromosomes, and chromosomes are the carriers of heredity.
The Chromosomal Theory of Inheritance states that:
- Genes are located on chromosomes in a linear fashion.
- Each parent contributes one set of chromosomes to the offspring, maintaining species-specific chromosome numbers.
- During meiosis, homologous chromosomes segregate and assort independently, just as Mendel described for "factors" (genes).
- Therefore, the behavior of chromosomes during gamete formation is the physical basis for Mendel’s laws of segregation and independent assortment.
- Since genes are arranged linearly on chromosomes, those on the same chromosome may be linked, showing deviations from Mendelian ratios.
Thus, chromosomes act as the vehicles of heredity, carrying genetic information from one generation to the next.
To support the chromosomal theory, several lines of evidence were provided through experimental studies in both plants and animals. Some of the most significant are:
- Both genes and chromosomes exist in pairs in diploid organisms.
- Genes segregate during gamete formation, and similarly, homologous chromosomes separate during meiosis I.
- Independent assortment of genes corresponds to the independent assortment of different chromosome pairs.
This close similarity between gene behavior (as predicted by Mendel) and chromosomal movement during meiosis strongly supported the theory.
Sex-linked inheritance provides direct evidence that genes are located on chromosomes. Thomas Hunt Morgan (1910), working on Drosophila melanogaster (fruit fly), discovered white-eye mutation, which was inherited only in males.
This was explained by the fact that the white-eye gene is located on the X chromosome. Since males have XY chromosomes, the presence of a mutant allele on the single X chromosome is expressed, whereas females (XX) can mask the effect with a normal allele.
Thus, sex-linked traits clearly showed the association between a gene and a specific chromosome.
Morgan and his students observed cases of non-disjunction in Drosophila. Normally, X chromosomes separate equally during meiosis, but sometimes they fail to do so (non-disjunction).
This resulted in abnormal offspring such as XXY females or XO males. The inheritance of traits in these flies correlated directly with the abnormal distribution of sex chromosomes.
This provided further evidence that genes reside on chromosomes.
In some Drosophila females, two X chromosomes were found attached together and behaved as a single unit during meiosis. The inheritance patterns of traits linked to these attached X chromosomes further confirmed the physical presence of genes on chromosomes.
Morgan’s group also studied the bar-eye mutation in Drosophila. This mutation showed that the intensity of the bar-eye trait depended on the duplication of a gene locus on the X chromosome.
Later, Morgan’s student, Alfred Sturtevant, created the first chromosome map by calculating the frequency of crossing over between linked genes. This showed that genes are arranged linearly on chromosomes and that crossing over leads to genetic recombination.
The chromosomal theory of inheritance provided a physical basis for heredity by linking Mendel’s abstract factors (genes) to concrete cellular structures (chromosomes). Evidence from sex linkage, non-disjunction, attached X chromosomes, and genetic mapping confirmed that genes are located on chromosomes and that their behavior during meiosis explains inheritance patterns.
This theory laid the foundation for modern genetics, molecular biology, and our current understanding of DNA as the ultimate genetic material.
