“Classical Experiments on Linkage, its Detection and Estimation”

 

UNIVERSITY OF AGRICULTURAL SCIENCES, BANGALORE
DEPARTMENT OF GENETICS AND PLANT BREEDING

Seminar II - GPB 582 (0+1)

“Classical Experiments on Linkage, its Detection and Estimation”

When Mendel observed an "Independent Assortment of traits" (Mendel's second law), he was fortunate to have chosen traits that were not localized close to one another on the same chromosome. Subsequent studies revealed that many genes were indeed linked, i.e., that traits did not assort or segregate independently, but that traits encoded by these linked genes were inherited together¹. In their 1905 dihybrid cross experiment, William Bateson, Edith Saunders, and Reginald Punnett expected the F₂ generation to follow the Mendelian 9:3:3:1 phenotypic ratio. However, they observed a significant excess of parental combinations and a marked deficiency of recombinant types. These deviations from Mendelian expectations provided the earliest experimental evidence of genetic linkage, suggesting that certain genes tend to be inherited together because they are located on the same chromosome. Studying Drosophila genetics, T. H. Morgan showed that the degree of linkage increased with physical proximity of the genes and that the 4 genetic linkage groups actually corresponded to the presence of 4 chromosomes in Drosophila¹.

Genetic linkage refers to the tendency for genes located on the same chromosome to be inherited together. When genes are situated on the same chromosome and are relatively close to each other, they are likely to be passed on as a unit during inheritance, rather than assorting independently. It is classified into complete and incomplete types based on the degree of recombination. Linkage can also be in the cis (coupling) phase, where dominant alleles are on the same chromosome, or trans (repulsion) phase, where dominant and recessive alleles are on opposite chromosomes².

Genetic map construction involves determining the linear order and relative distances between genes on a chromosome based on recombination frequencies. The frequency of recombination between two genes reflects their physical distance with 1% recombination equivalent to 1 centiMorgan (cM). Further, using an F₂ population of foxtail millet (Jingu28 × Ai88), a linkage map was constructed and 46 QTL related to 12 agronomic traits were identified. Two major genes associated with plant height Seita.5G404900 and heading date Seita.9G020100 were discovered, enabling precise genetic improvement.

By analysing the progeny of test crosses or mapping populations, such as F₂, backcross, or recombinant inbred lines, scientists estimate linkage distances and construct genetic linkage maps. These maps are essential for identifying gene locations, marker-assisted selection, and understanding genome structure³.

REFERENCES

1 PULST, S.M., 1999. Genetic linkage analysis. Arch. Neurol., 56(6): 667-672.
2 RUSSELL PJ. 2010. iGenet.: Mol. Appro., 3rd ed. San Francisco: Benjamin Cummings., 152–156.
3 SINGH BD. 2019. Genetics. 11th ed. New Delhi, India: Kalyani Publishers., 458–463.