Question
What is the chromosomal theory of inheritance? What evidence did Sutton and Boveri provide? How did Morgan’s experiments with Drosophila prove that genes are on chromosomes?
(NEET + CBSE Class 12)
Solution — Step by Step
In 1902, Walter Sutton and Theodor Boveri independently proposed that Mendel’s “factors” (genes) are located on chromosomes. Their reasoning was based on striking parallels between the behaviour of chromosomes during meiosis and the behaviour of Mendel’s factors during inheritance.
| Mendel’s factors | Chromosomes |
|---|---|
| Occur in pairs (alleles) | Occur in pairs (homologous) |
| Separate during gamete formation | Separate during meiosis I (anaphase I) |
| Independent assortment of different pairs | Independent assortment of non-homologous chromosomes |
| One from each parent in offspring | One homologue from father, one from mother |
These parallels were too precise to be coincidence — genes must physically reside on chromosomes.
T.H. Morgan (1910) provided experimental proof using fruit flies. He found that:
- The gene for eye colour in Drosophila is located on the X chromosome
- White-eyed males passed the trait differently than autosomal genes (sex-linked inheritance)
- This could only be explained if the gene was physically on a specific chromosome
Morgan also showed linkage — genes on the same chromosome tend to be inherited together, deviating from Mendel’s law of independent assortment. The degree of linkage depends on the physical distance between genes.
Evidence Flowchart
flowchart TD
A["Mendel's Laws — 1860s"] --> B["Factors behave in predictable patterns"]
C["Meiosis discovered — 1890s"] --> D["Chromosomes behave similarly to Mendel's factors"]
B --> E["Sutton and Boveri — 1902"]
D --> E
E --> F["Chromosomal Theory: genes are on chromosomes"]
F --> G["Morgan's Drosophila experiments — 1910"]
G --> H["Sex-linked inheritance proves X-linked genes"]
G --> I["Linkage proves genes on same chromosome travel together"]
H --> J["Theory confirmed experimentally"]
I --> JWhy This Works
The chromosomal theory of inheritance was built on logical parallelism. Mendel described abstract “factors” without knowing where they sat. When cytologists observed chromosomes behaving exactly as Mendel’s factors should, the connection became obvious.
Morgan’s Drosophila work was the clincher because it tied a specific gene (white eye colour) to a specific chromosome (X), and showed that genes on the same chromosome are inherited together (linkage) — something Mendel never observed because his pea traits happened to be on different chromosomes.
Common Mistake
Students confuse linkage with Mendel’s law of independent assortment. Independent assortment applies only when genes are on different chromosomes. Linked genes (on the same chromosome) are inherited together and do NOT assort independently. Recombination (crossing over) can break linkage, but only partially — the closer the genes are, the stronger the linkage. NEET asks: “When does Mendel’s law of independent assortment NOT hold?” Answer: when genes are linked (on the same chromosome).