Thomas Hunt Morgan:

Experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variation that sexual reproduction produced.

Morgan worked with the tiny fruit files, Drosophila melanogaster because-

• They could be grown on simple synthetic medium in the laboratory.

• They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies.

• There was a clear differentiation of the sexes the male and female flies are easily distinguishable.

• It has many types of hereditary variations that can be seen with low power microscopes

T. H. Morgan’s EXPERIMENT:

Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked.
Genetic linkage was first discovered by the British geneticists William Bateson and Reginald Punnett shortly after Mendel’s laws were rediscovered. The understanding of genetic linkage was expanded by the work of Thomas Hunt Morgan. Morgan’s observation that the amount of crossing over between linked genes differs led to the idea that crossover frequency might indicate the distance separating genes on the chromosome. This distance is expressed in terms of a genetic map unit (m.u.) or a centimorgan and is defined as the distance between genes for which one product of meiosis in 100 is recombinant. A recombinant frequency (RF) of 1% is equivalent to 1 m.u.
Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny.
He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the 9:3:3:1 ratio (expected when the two genes are independent).

Conclusion:

• The genes for these two characters yellow-bodied, white-eyed in females and brown-bodied, red-eyed in males were situated on the same chromosome(x- chromosome).
• The proportion/frequency of parental gene combinations was much higher than the non-parental typedue to the physical association or linkage of the two genes.
• some genes present on same chromosome were very tightly linked (showedvery low recombination) while others were looselylinked (showed higher recombination)
• Forexample he found that the genes for white eye and yellow body colourwere very tightly linkedand showed only 1.3 per cent recombination while white eye and miniaturewing showed 37.2 per cent recombination.

Linkage and Recombination:

Linkage – physical association of genes on a chromosome OR The tendency of two or more genes in a chromosome to remain together during inheritance
Incomplete linkage: When Linkage is broken due to crossing over and some recombinant forms appear
Recombination – generation of non-parental gene combinations
Linked genes: The genes located on the same chromosome are said to be linked.

Linkage map:

A linkage map is a genetic map of a species that shows the position of its known genesrelative to each other.A genetic map is based on the frequencies of recombination between genes during crossing over of homologous chromosomes. The greater the frequency of recombination (segregation) between two genes, the farther apart they are assumed to be. The lower the frequency of recombination between the genes smaller the physical distance between them.

Crossing Over:

Mutual exchange of chromosome parts between two non-sister chromatids of a homologous pair of chromosomes.
Due to crossing over linked genes are separated and enter into different gametes.
Crossing over takes place during gamete formation at pachytene stage of meiosis
It occurs randomly at one or more than one points called chiasmata along the whole length of homologous pair of chromosomes.