for BSc II students

Linkage

Chromosomes take part in the transmission of hereditary characters through hereditary units or genes. An individual has usually many genes. So, we can say that one chromosome contains more than one gene. The genes for different characters may be either situated in same chromosome or in different chromosomes. If genes are situated in the same chromosome and are fairly close to each other then they tend to be inherited together. This type of coexistence of two or more genes in the same chromosome in known as linkage

The hypothesis that linked genes tend to remain in their original combination due to close location in the same chromosome was advanced by T H Morgan in 1911. Before Morgan, Sutton 1903 and Bateson and Punnet 1906 had given some hints about the phenomenon of linkage.



Types of linkage

T H Morgan and his co-workers by their investigations on the Drosophila and other organisms have found two types of linkage.
1. Complete linkage.
2. Incomplete linkage.


The complete linkage is the phenomenon in which parental combinations of characters appear together for two or more generations in a continuous and regular basis. In this case, the linked genes are closely associated and tend to transmit together.

Example The genes for bent wings (bt) and shaven bristles(svn) of the fourth chromosome (mutant) of Drosophila melanogaster exhibit complete linkage.

The linked genes do not always stay together because homologous non- sister chromatids may exchange segments of varying length with one another during meiotic prophase. This sort of exchange of chromosomal segments in between homologous chromosomes is known as crossing over. The linked genes which are widely located in chromosome and have chances of separation by crossing over is called incompletely linked genes and the phenomenon of their inheritance is called incomplete linkage.

Example linked genes for body color, black body (b) and grey body + and wing shape, vestigial (v) and long wing +

Linkage groups

All the linked genes of a chromosome form a linkage group. Linkage groups of a homologous chromosome are considered as one. The linkage group of a species corresponds with the haploid chromosome number of that species.

Example Drosophila has 4 pairs of chromosomes and 4 linkage groups
Man has 23 pairs of chromosomes and 23 linkage groups

Significance of linkage

The phenomenon of linkage has one of the great significance for the living organisms that it reduces the possibility of variability in gametes unless crossing over occurs.