Sunday, July 26, 2020

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

5. PRINCIPLES OF INHERITANCE AND VARIATION

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 chromosomal theory of inheritance using fruit flies (Drosophila melanogaster).

It is the 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. Male is smaller than 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      X      Brown-bodied, red-eyed males (wild type)

Cross 2: White-eyed, miniature winged      X      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.

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