Principles of Inheritance and Variation - Notes | Class 12 | Part 5: Chromosomal Theory of Inheritance, Morgan's experiment

Chromosomal Theory of Inheritance

• Mendel’s work remained unrecognized till 1900 because:
  • Communication was not easy.
  • His mathematical approach was new and unacceptable.
  • The concept of genes (factors) as stable and discrete units could not explain the continuous variation seen in nature.
  • He could not give physical proof for the existence of factors.
• In 1900, de Vries, Correns, & von Tschermak independently rediscovered Mendel’s results.

Chromosomal Theory of Inheritance (1902)

• Proposed by Walter Sutton & Theodore Boveri.
• They said that pairing & separation of a pair of chromosomes lead to segregation of a pair of factors they carried.
• Sutton united chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance. It states that:
  • Chromosomes are vehicles of heredity.
  • Two identical chromosomes form a homologous pair.
  • Homologous pair segregates during gamete formation.
  • Independent pairs segregate independently of each other.
• Genes (factors) are present on chromosomes. Hence genes and chromosomes show similar behaviours.

• Thomas Hunt Morgan proved the chromosomal theory of inheritance using fruit flies (Drosophila melanogaster).
• It is a suitable material for genetic study because:
  • They can grow on simple synthetic medium.
  • Short generation time (life cycle: 12-14 days).
  • Breeding can be done throughout the year.
  • Hundreds of progenies per mating.
  • Male and female flies are easily distinguishable. E.g., the male is smaller than the female.
  • It has many types of hereditary variations that can be seen with low-power microscopes.

Linkage and Recombination

• Linkage is the physical association of two or more genes on a chromosome. They do not show independent assortment.
• Recombination is the generation of non-parental gene combinations. It occurs due to independent assortment or crossing over.
• Morgan carried out several dihybrid crosses in Drosophila to study sex-linked genes. E.g.:
  • Cross 1: Yellow-bodied, white-eyed females × Brown-bodied, red-eyed males (wild type).
  • Cross 2: White-eyed, miniature-winged × Red-eyed, large-winged (wild type).
• Morgan intercrossed their F1 progeny. He found that:
  • The two genes did not segregate independently, and the F2 ratio deviated from the 9:3:3:1 ratio.
  • Genes were located on the X chromosome.
  • When two genes were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type. This is due to linkage.
  • Genes of white eye & yellow body were very tightly linked and showed only 1.3% recombination.
  • Genes of white eye & miniature wing were loosely linked and showed 37.2% recombination.
  • Tightly linked genes show low recombination. Loosely linked genes show high recombination.

• Alfred Sturtevant used the recombination frequency between gene pairs for measuring the distance between genes and ‘mapped’ their position on the chromosome.
• Genetic maps are used as a starting point in the sequencing of genomes. E.g., Human Genome Project.

Select a Topic 👇

Topic 1: Mendel's Experiments Topic 2: Inheritance of One Gene Topic 3: Inheritance of Two Genes Topic 4: Other Patterns of Inheritance Topic 5: Chromosomal Theory, Morgan's Experiment Topic 6: Sex Determination Topic 7: Mutation and Pedigree Analysis Topic 8: Genetic Disorders (Mendelian and Chromosomal)

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