Definition : The genes located on sa the e chromosome are said to be linked genes, they remain linked together is passing referred as Linkage.
Introduction : According to principle of Independent Assortment, when two or more genes are considered together, they assort (separate) randomly during distribution into gametes.
But later, it was argued that Mendel second preneeble of Independent Assortment is not universal but is limited to genes located in different chromosome only. Sutton (1903) pointed out that number of genes in most organism exceeds number of pairs of chromosomes (ex : Drosophila have 4 pairs, pea has 7 pairs, human have 23 pair of chromosomes)
Therefore, It is likely that each chromosome must contain many genes, such genes which are present on same chromosome would not show same Independent Assortment which characterizes genes located on different pairs of chromosome . But they would be linked together in heredity by undue of their common chromosome. Thus, all genes on one chromosome are said to be linked genes.
Subtitle – 1 :
Three years later, (1906) two English Geneticist Batson and Punnet demonstrated Phenomenon of linkage in sweet pea.
(They found two paris of alleles which did not assort independently) Here, a pair of genes which influences, the color of flavor (Purple/Red) was linked to another pair which influences shape of pollen grain (log/Round). (Here, purple and Long are dominant white Red and Round are recessive alleles)
Subtitle – 2 :
Plants with purple Flower and Long pollen grain were crossed with Red flower and round pollen grain
A test cross was made on F1 generation plant, and it was found that instead of Independent seguegation of these two qualities (Purple and Long). The color of flower and pollen shape tended to remain together as in parental type. Actual ration obtained was 7 : 1 : 1 : 7 instead of 1 : 1 : 1 : 1 (which would be expected if genes have independently assorted.
Conclusion : This unbalanced ratio led Batson and Punnet to suspect that thex two pairs of genes must lie on same chromosome.
( However, there is some recombination but it is much less frequent in linked genes than in non – linked genes).