Linkage

Linkage

The phenomenon of permanent association of genes of a single chromosome that can be inherited in successive generations in same position and proportion without any changes or separation is called linkage.

Mendel did not notice the phenomenon of linkage because he chose characteristics controlled by genes located on different chromosomes. This phenomenon of linkage was noticed by post- Mendelian geneticists. It was firstly discovered by Bateson and Punnet (1906) while working on sweet pea (Lathyrus odoratus)in which they found two pairs of alleles did not assort independently. Sutton and Boveri first suggested linkage in 1903 when they first suggestion linkage when they propounded "Chromosomal Theory of Inheritance." The term 'Linkage' was coined by T.H. Morgan and he proposed "chromosomal theory of linkage" by working on Drosophila 1910. According to this theory, the linkage is defined as the phenomenon of inheritance of genes together and retain their parental combination even in the offspring.

Linked and unlinked genes

The genes present in the single chromosome are inherited together are called linked genes. Unlinked genes are those genes which are found on the different chromosome. Those characters which are controlled by linked genes are called linked characters.

Linkage group

Genes situated on the chromosome are said to be linked. The genes on the single chromosome form a linkage group. A linkage group usually passes into a gamete and are inherited together. Linkage group in a cell equals to the pair of chromosomes present in the cell of an organism. But it should be noted that the number of linkage group is restricted to the haploid number of chromosomes of an organism. The total number of linked genes present in a single chromosome from the linkage group. The linkage group can be determined by:

number of linkage group= number of haploid chromosomes in an organism,

Example- In Drosophila-2n = 8⇒ n=4⇒ n= 3+xy⇒ 3+x+y⇒ 5

In human female- 2n=46⇒ n=23⇒ n=22+xx⇒ 22+x (homomeric/gametic) ⇒ 23

Chromosomal Theory of Linkage-

Morgan formulated the chromosomal theory of linkage according to which-

1)Genes lie in linear order in the chromosomes and distance between them is variable.

2)The genes that are linked, stay on the same chromosome.

3)The tendency of genes to remain together in their parental combination is due to their presence on the same chromosome.

4)The strength of linkage is directly related to the distance between the linked genes on a chromosome. The closer the genes are located, stronger is the linkage.

Types of linkage

1) Complete linkage

2) Incomplete linkage

1) Complete linkage -The phenomenon in which the genes present in a single chromosome do not separate and are inherited together in the successive generations due to the absence of crossing over is called complete linkage. Hence it produces only parental combination but not non-parental ones. It is very rare in nature.

Example :

A cross between Drosophilawith gray body- normal wings [GGNN] and black body-vestigial wings [ggnn] -

The above cross produces F₁ hybrids with the gray body- normal wings [GgNn]. When these F₁ hybrids are crossed with the recessive parent having a black body- vestigial wings (test-cross), it produces two types of offspring in equal proportion (50% - 50%) in F₂ generation. These F₂ offspring resemble their grandparents.

Hence, the gray-body character is inherited with normal wings and black body character is inherited together with vestigial wings. It means these genes are linked genes and no non-parental combination are formed due to the absence of crossing over.

From the above cross - F₂ ratio= gray body normal wings= 50%

and, black body vestigial wings= 50%

2) Incomplete linkage -The phenomenon in which linked genes present in the same chromosome have a tendency to separate due to crossing over and forms both parental and non-parental combination in the F₂generation is called incomplete linkage. It can be illustrated in maize grains.

When colored-full seed [CCFF] of maize is crossed with colorless-shrunken seed [ccff] of maize-

The above cross produces F₁hybrids with coloured-full seeds [CcFf] of maize. When F₁ hybrid female is crossed with recessive parent i:e colorless shrunken male, they produce both parental and non-parental combination with four phenotypes in the ratio of 1:1:1:1 i:e 1 colored-full : 1 colored-shrunken : 1 colorless-full : 1 colorless-shrunken seed. This is possible due to the incomplete linkage.

From the cross above the F₂ phenotypic ratio is = 1:1:1:1.

Significance of linkage-

The phenomenon of linkage has great significance for living organisms, because-

1) It reduces the possibility of variability in gametes.

2) The number of linkage groups is equivalent to number of chromosomes present in genome,

3) It is useful for maintaining the good characters in new variety,

4) Linked character is preserved for successive generations because linkage prevents the incidence of crossing over.

5) It doesn't permit the plant breeders to bring the desirable character in a variety,

Reasons for selectingDrosophilafor genetic studies-

The reasons for selectingDrosophilafor genetic studies are as follows-

1)Drosophilacan be cultured easily under normal condition.

2)Drosophilais harmless and inexpensive to culture. The culture medium required to rear them is also very simple.

3) A large number of Drosophilacan be accommodated in a small space.

4) Progeny produced after each mating is large.

5) It breeds throughout the year. Its life cycle is short and completed in 10-12 days so that the results of controlled breeding are quickly available.

6) The number of chromosomes is only four pairs and all are of different size and shape. Chromosomes II, III, and IV are autosomes while the chromosome 'I' is sex chromosome. Female has XX and male has XY, Y being characteristically J-shaped. It is, therefore, easy to identify and study each chromosome.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.