Tendency of  genes of a chromosome to remain together is called linkage and their genes are called linked genes.The unit for measuring the distance between the two genes is Centimorgan . 



                     Complete linkage – In this type of linkage, genes are closely situated.So,                                                                                        there is no possibility of separation.It is rare.

                            Incomplete linkage  In this type of linkage,genes are far apart.So,they                                                                                               have great chances of separation. 


                       Sutton’s view on linkage In 1903, SUTTON stated that genes are arranged on chromosome in linear order.During meosis , chromosomes move as a unit.So, all the genes of a chromosome are linked. SUTTON could not prove his view.

                         Coupling and repulsion theory – It was given by BATESON and PUNNETT (1906).They crossed pure blue and long pollen grain sweet pea plant with pure red and round pollen grain pea plant.In F1 generation blue colour and long pollen grain appeared.When a test cross was done 7:1:1:7 ratio was obtain but according to Mendel it should be 1:1:1:1.

                          7:1:1:7 ratio indicates that dominant alleles tend to remain together. BATESON and PUNNETT called it coupling . Recessive alleles tend to remain separate. It was called  repulsion .

                   Pure blue and long pollen grain       x          Pure red and round pollen grain


                                  BBLL                                                                 bbll    



                                                                                BbLl                                                                                F1

                                                                   Blue and long pollen grains

                                           BbLl                      TEST CROSS                      bbll

F1 hybrid                             x                                     Recessive parent 

                                               ↓                                                                                                 7      :        1        :     1        : 7                                   

                                                        blue & long     blue & round   red & long   red & round                                   F2

Morgan’s view on linkageT .H.Morgan stated in 1910 that coupling and repulsion are two sides of a phenomenon which is called linkage.He stated that genes of  homozygous  parent remain together while the genes of  heterozygous parents remain separated. 

                     Chromosomal theory of linkageIt was proposed by Morgan and Castle . According to him – 

                        1. Genes are arranged on chromosome in linear fashion.

                        2. The gene which show linkage are found on some chromosome and surrounded by chromosome material.So, they can’t separated.

                        3.There is a strong linkage in closely situated genes while there is weak linkage in genes situated apart.

                                          SIGNIFICANCE OF LINKAGE    

Linkage reduces the chances of  separation of genes. So, new combinations do not arise in next generation.It also reduces the chances of variation. Linkage has also helped in maintaining parental character together.In nature it checks the appearance of new combination.


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