LINKAGE
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 .
TYPES OF LINKAGE –
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.
THEORIES OF LINKAGE –
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
P1
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 linkage – T .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 linkage – It 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.