Linkage Group

Linkage can be defined as the tendency of certain genes to stay together during chromosomal inheritance. The genes that are inherited together and located on the same chromosome are called linked genes and the genes that are located on different chromosomes and show independent assortment are called non-linked genes. Linked genes do not show independent assortment at the time of gamete formation. 

Linkage was first discovered by Bateson and Punnett in 1906 in sweet pea. Although, the term linkage was first coined by Morgan.

What is a Linkage Group?

In order to define linkage groups or specifically define linkage in biology, one has to understand that genes are located on the chromosomes. These genes can be specific markers that are located on the chromosomes. These also result in certain phenotypes i.e. physical characteristics such as long, short, round, rough, etc. 

These genes, according to Mendel’s Laws of Inheritance, are typically known to assort independently of each other. But some of the phenotypes are known to be combined together as they appear in the species along with one another. Genes that are present on a homologous pair of chromosomes are known as linkage groups and they inherit together except for crossing over. The number of linkage groups present in an organism corresponds to  the haploid number of chromosomes in the organism. For example, Drosophila melanogaster consists of 4 pairs of chromosomes and thus has 4 linkage groups. When two or more genetic markers are present physically near to each other on a chromosome and are highly unlikely to be separated on different chromatids when there is chromosomal crossover while the cell is undergoing meiosis cell division, they are said to be linked together with one another.

Linkage Group in Biology

The discovery of linkage groups clarified the reason why certain traits are usually seen to be inherited together. This work provided proof of the concept that genes are physical structures that are related by a unit of physical distance. 

This unit of physical distance is centimorgans (cm). A distance of 1 cm is said to be the separation of two different markers per 100 meiotic product or 50 meiosis cycle. These linkage groups and linkage concepts are used to construct linkage maps that show the relative distances between two markers. 

Linkage Maps and Linkage Analysis

A linkage map is also known as a genetic map. Such a map is a tabular representation of a species or experimental population which states that the position of the known genes or genetic markers is relative to each other in terms of frequency of recombination instead of a specific physical distance along each of the chromosomes. One of the first such linkage maps to be developed was prepared using the linkage group in Drosophila. A linkage map is prepared on the basis of the frequencies of the recombination event between two or more markers during the crossing over of the homologous chromosomes. 

Based on the concepts that define linkage in biology, there is a method of linkage analysis that is used to search for the segments of chromosomes that usually segregate together with a specific phenotype through the generations of the same family. Linkage analysis can also be used to determine the linkage maps in cases of both binary and quantitative traits. But there are certain limitations to the method of linkage analysis. 

Although the linkage analysis has been successful in identifying genetic variants in human beings, via the different number of linkage groups in humans, that are the cause of rare disorders like Huntington’s disease, it has failed itself when it is applied for more common disorders like heart disease and different forms of cancer. An explanation for this sort of occurrence is that the genetic mechanisms that play a part in common disorders are different from the mechanisms that play a role in rare disorders. 

Common Example of Linkage Groups - Sex Linkage

Sexual phenotypes or sexual characteristics are one prominent example that can be used to state linkage and linkage groups. This concept of sex linkage can explain the linkage group in human male and female and provide explanations for the characteristics to be transferred and carried as linkage groups. Sex linkage is the concept in which certain characteristics or phenotypes can be linked to one sex. The complete set of genes of the X-chromosome is carried together in both human beings and Drosophila flies, while the Y-chromosomes carry only a few genes together. Hence, the linkage group in human males is relatively small as compared to the linkage group in human females. 

It is well-established that the eggs of the female carry the X-chromosome and the sperm cells may carry either X-chromosome or Y-chromosome. When an egg carrying an X-chromosome is fertilised by a sperm carrying another X-chromosome a female is born, and when an egg is fertilised by the sperm carrying a Y-chromosome a male is born. Hence, in a child carrying an XY chromosome pair, any phenotype or trait that is carried by the X-chromosome will be expressed unless and until a corresponding allele is present on the Y-chromosome. 

Examples of sex-linked traits in males that follow the linkage group in human males are red and green colour blindness and haemophilia. This is because the phenotypes are controlled by the genes present on the X-chromosome and have a higher frequency of occurrence in males than females because of the absence of a corresponding allele on the Y-chromosome. 

Linkage and Crossing Over

Linkage can be described as the tendency of genes in a chromosome to stay together during chromosomal inheritance. Whereas crossing over can be defined as the exchange of genetic materials between the homologous chromosomes that results in a new combination of genes.

Linkage produces parental types and helps to maintain a new improved variety. On the contrary, crossing over results in recombinations and plays a crucial role in evolution. 

Significance of Linkage

The significance of linkage is given below:

• It helps to restore the parental genes for the coming future generations.

• It is useful to maintain the good characters of a newly developed variety.

• It plays a pivotal role in determining the hybridisation scope of a particular plant.