What are the linked genes?

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About Linkage of genes 

When genes are closely present link together in a group and transmitted as a single unit, the is phenomenon is called linkage. For more details about the Biology you can refer zoology class 12 Notes prepared by academic team of Physics Wallah. 

Theories of linkage

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

(ii) 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.

(iii) 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   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   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).  (iv) 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 : 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.

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What are the linked genes?

What are the linked genes?

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