Chromosomal theory

🧬 Chromosomal Theory of Inheritance

A complete, animated study guide for biology learners

1. Introduction

The Chromosomal Theory of Inheritance connects Mendel’s laws of heredity to chromosomes, which carry genes and pass them to offspring through gametes.

2. History & Discoveries

Scientist	Year	Contribution
Walter Flemming	1882	Discovered chromosomes (“coloured bodies”).
Walter Sutton	1902	Linked chromosomes with Mendel’s factors.
Theodor Boveri	1902	Confirmed chromosomes carry heredity.
Carl Correns	1890	Rediscovered Mendel’s work.

3. Sutton & Boveri’s Chromosomal Theory

Genes are located on chromosomes, and chromosomes carry hereditary information to offspring through gametes.

Evidences Supporting the Theory

1. Equal Contribution: Both sperm and egg contribute equally. Hereditary material lies in the nucleus where chromosomes are present.

2. Segregation: Chromosomes separate during meiosis just like Mendel’s factors (law of segregation).

3. Haploid Gametes: Gametes have half the chromosome number; fertilization restores diploid number.

4. Independent Assortment: Chromosomes line up randomly at metaphase, creating variation — matching Mendel’s law.

4. Problem in the Theory

Scientists noticed more independent traits than chromosome pairs. The mystery was solved when it was discovered that each chromosome carries many genes. Genes close together show linkage; those farther apart separate by crossing over.

5. T. H. Morgan’s Experimental Proof (1910)

Using Drosophila melanogaster (fruit fly), Morgan found a white-eyed male mutant and performed cross experiments.

• Red-eyed female × white-eyed male → F₁: all red-eyed.
• White-eyed female × red-eyed male → F₂: only males had white eyes.

Conclusion: The eye colour gene is located on the X chromosome — a sex-linked gene.

Other Discoveries

• Linkage: Genes on the same chromosome are inherited together.
• Crossing Over: Exchange of parts between homologous chromosomes creates new combinations.
• Genetic Mapping: Distance between genes estimated by crossing-over frequency.

6. Final Conclusions

• Genes are located on chromosomes.
• Chromosomes carry hereditary information.
• Meiosis separates and recombines genes.
• Fertilization restores diploid number.
• Independent assortment and crossing-over produce variation.

7. Key Terms

Term	Meaning
Gene	Unit of heredity controlling a specific trait.
Chromosome	Thread-like DNA-protein structure carrying genes.
Homologous Chromosomes	Same genes; one from each parent.
Diploid	Two sets of chromosomes (2n).
Haploid	One set of chromosomes (n).
Meiosis	Cell division forming haploid gametes.
Independent Assortment	Random separation of chromosome pairs.
Linkage	Genes on same chromosome inherited together.
Crossing Over	Exchange of chromosome parts creating new gene combinations.

8. Quick Revision Summary

Topic	Key Idea
Discovery	Flemming (1882)
Theory	Sutton & Boveri (1902)
Experimental Proof	T. H. Morgan (1910)
Concepts	Linkage, Crossing Over, Genetic Mapping

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