What was the criticism of chromosomal theory of Sutton and Boveri? How was it resolved?

The Chromosomal Theory of Inheritance was independently proposed by Walter Sutton (1902) and Theodor Boveri (1902–1903). This theory stated that genes are physically located on chromosomes and the behavior of chromosomes during meiosis explains Mendel's laws of segregation and independent assortment.

Criticism of Chromosomal Theory of Sutton and Boveri

Although this theory provided a clear cytological basis for heredity, it initially faced several criticisms, as the scientific community was not fully convinced without direct evidence. There were three major criticisms raised against this theory:

1. Lack of Direct Evidence That Genes Located on Chromosomes

When Sutton and Boveri proposed the theory, there was no direct experimental proof showing that genes were located on chromosomes. While chromosomes could be seen under a microscope, genes were hypothetical entities at that time. Critics argued that just because chromosomes and genes behave similarly during meiosis does not mean they are the same. This criticism focused on the lack of physical or molecular linkage between genes and chromosomes.

2. Number of Genes v/s Number of Chromosomes

Another major concern was the huge mismatch between the number of traits (genes) and the number of chromosomes. For example, Mendel had described several independent traits, and in humans and many organisms, thousands of traits were known. But the total number of chromosomes was very small (e.g., only 4 pairs in Drosophila, 23 pairs in humans). Critics asked, how can so many genes be accommodated on such few chromosomes? This created doubt whether chromosomes could really carry all genetic material.

3. Contradiction with Mendel's Law of Independent Assortment

According to Mendel's second law, genes for different traits assort independently. However, if multiple genes are present on the same chromosome, they should be inherited together. At that time, the idea of linkage and recombination was not understood. So, critics asked, if genes are on chromosomes, why do they still assort independently as Mendel observed? This was seen as a contradiction between Mendel's results and chromosome behavior.

Resolution of the Criticisms

These criticisms were addressed and resolved by the pioneering work of Thomas Hunt Morgan and his students, who worked extensively on Drosophila melanogaster at Columbia University in the early 20th century.

1. Proof That Genes Are on Chromosomes

In 1910, Thomas Hunt Morgan discovered sex-linked inheritance of eye color in Drosophila. He showed that the gene for eye color was located specifically on the X chromosome, providing direct evidence that genes reside on chromosomes. This was the first experimental confirmation of the chromosomal theory.

2. Concept of Gene Linkage and Crossing Over

To explain how multiple genes on the same chromosome could sometimes assort independently, Morgan and his student Alfred Sturtevant introduced the concept of linkage and crossing over. In 1913, Sturtevant created the first genetic map of the X chromosome based on recombination frequency. He showed that genes are linearly arranged on chromosomes and during meiosis, crossing over causes exchange of segments, allowing even linked genes to segregate independently in some cases.

This answered the earlier contradiction with Mendel's law of independent assortment. It was now understood that genes far apart on the same chromosome recombine frequently, appearing to assort independently.

3. Multiple Genes per Chromosome

Once linkage maps were developed, scientists realized that each chromosome can carry many genes in linear order. So even with few chromosomes, organisms can have thousands of genes. This resolved the doubt about the limited number of chromosomes.

Final Conclusion 

  • By the 1920s, the chromosomal theory of inheritance was fully accepted due to the experimental confirmation provided by Morgan and his team. The theory became the foundation of modern genetics. Every criticism that was initially raised was addressed scientifically through observations, mapping and molecular-level understanding.







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