Describe linkage and recombination in detail.
 

Morgan and his group observed in Drosophila that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combination were much higher than the non parental type.

They attributed this due to physical association of the two genes and coined the term 'linkage' to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non parental gene combinations.

Thus linkage is a phenomenon of genic inheritance in which gene of a particular chromosome show their tendency to inherit together.

Morgan and his group also found that even when gene were grouped on the same chromosome, some genes were tightly linked i.e. linkage is stronger between two genes if the frequency of recombination is low.

Whereas the frequency of recombination is higher if genes are loosely linked i.e. linkage is weak between two genes.

Recombination of linked genes is by crossing over (exchange of corresponding parts between the chromatids of homologous chromosomes).

All the genes linked together in a single chromosome constitute a linkage group.

The number of linkage group in an organism is equal to there haploid number of chromosomes.

This hypothesis was proved by $TH$ Morgan by his experiments on Drosophila.

Morgan and his group hybridised yellow bodies and white eyed females with brown bodies and red eyed males (wild type) and intercrossed their Fi-progeny (cross-$A$).

It was observed that the two genes did not segregate independently of each other and the $\mathrm{F}_{2}$ ratios deviated significantly from $9: 3: 3: 1$ ratio.

In $\mathrm{F}_{2}$ generation parental combinations were $98.7 \%$ and recombinant were $1.3 \%$.

In another cross (cross-B), between white bodies fly with miniature wing and a male fly with yellow body and normal wing, parental combinations were $62.8 \%$ and recombinants were $37.2 \%$ in $\mathrm{F}_{2}$ generation.

Thus it was proved from the crosses that the linkage between genes for yellow body and white eyes is stronger than the linkage between the white body and miniature wing.

His student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome.