12th Class Biology Principle Of Inheritance And Variation Linkage

Introduction : "When genes are closely present link together in a group and transmitted as a single unit this phenomenon is called linkage".

Theories of linkage

Sutton's hypothesis of linkage (1903) : The number of groups of genes are equivalent to the number of chromosomes.

Morgan's hypothesis of linkage (1910) : It was given by T. H. Morgan. According to him the genes of homologous parents enter in the same gamete and tend to remain together, which is opposite in heterozygous parents. Linked group are located on the same chromosome and distance between linked group of gene limits the grade of linkage.

Coupling and repulsion hypothesis : Proposed by Bateson and Punnet (1906) that dominant alleles tend to remain together as well with recessive alleles, called gametic coupling. If dominant and recessive alleles are present in different parents they tend to remain separate and called repulsion. When BBLL and bbll are crossed, the \[{{F}_{1}}\] is BbLl and the test cross of it will show progeny in 7 : 1 : 1 : 7 ratio i.e., BbLl : Bbll : bbLl : bbll (coupling) when BBll is crossed with bbLL the \[{{F}_{1}}\] is BbLl or the test cross progeny will show 1 : 7 : 7 : 1 ratio i.e., BbLl : Bbll : bbLl : bbll (repulsion). Coupled and repulsed genes are known as linked genes. Linkage has coupling phase and repulsion phase. In coupling phase both the linked genes have their dominant alleles in one chromosome and recessive alleles in other chromosomes. The heterozygotes with such constitution is called cis heterozygote. Cis-arrangement is a original arrangement. Which form two types of gametes as (AB) and (ab). In Human X–chromosomes carry 102 genes and Y chromosome carries 10 genes only.

In repulsion phase the normal alleles as well as mutant alleles lie in opposite chromosomes of the homologous pair, such heterozygote is called as trans heterozygote. It is not original arrangement, caused due to crossing over, which form two types of gametes as (Ab) and (aB).

Chromosomal hypothesis of linkage : It was given by Morgan and Castle. According to them linked genes are bound by chromosomal material and are transmitted as a whole.

Types of linkage

Depending upon the absence or presence of nonparental or new combination of linked genes, linkage has been found to be complete or incomplete.

Complete linkage (Morgan, 1919) : Such cases in which linked genes are transmitted together to the offsprings only in their original or parental combination for two or more or several generations exhibit complete linkage. In such cases the linked genes do not separate to form the new or non-parental combinations. This phenomenon is very rare. Some characteristics in males of Drosophila are found to exhibit complete linkage.

Incomplete linkage : In majority of cases, the homologous chromosomes undergo breakage and reunion during gametogenesis. During reunion the broken pieces of the chromatids are exchanged, producing some nonparental or new combinations. Therefore, the linkage is rendered incomplete. The phenomenon of interchange of chromosome segments between two homologous chromosomes is called crossing over. Incomplete linkage is very common and has been studied in almost all the organisms. Hutchinson discribe incomplete linkage in maize seed.

Linkage groups

All the genes which are linked with one another, form a linkage group. Since linked genes are present in the same chromosome, the number of linkage group in an animal or plant is equal to the haploid number of chromosomes present in its cells. This hypothesis was given by Sutton and was proved by experiments on Drosophila by T.H. Morgan.

Strength of linkage

The strength of linkage between any two pairs of linked genes of a chromosome depend upon the distance between them. Closely located genes show strong linkage, while genes widely located show weak linkages.

\[Strength\text{ }of\text{ }linkage\propto \frac{1}{Dis\tan ce\,between\,the\,gene}\].

Factor affected to linkage

Linkage is affected by the following factors :

Distance : Closely located genes show strong linkage while genes widely located show weak linkage.

Age : With increasing age the strength of linkage increases.

Temperature : Increasing temperature decreases the strength of linkage.

X-rays : X-rays treatment reduces the strength of linkage.

Significance of linkage

(i) It helps in maintaining the valuable traits of a newly developed variety.

(ii) It helps locating genes on chromosome.

(iii) It disallows the breeders to combine all the desirable traits in a single variety.