Linkage

The tendency of genes do not change their location, that is to remain their original position during inheritance and are found on the chromosome at the same stage and location as the parental chromosomes (first generation).

The above phenomenon is called linkage and the genes which are showing linkage are called linked gene.

Therefore, when genes are transferred from one generation to another, there is no change in their location and they will be present at the same place or loci where they were present in the parent.

Linkage-Historical Aspects

The hypothesis of linkage was first given by Sutton & Boveri (1902-03).

But Linkage was experimentally proved by TH Morgan (Thomas Hunt Morgan-1910) when he was studying Drosophila.

To understand the process of linkage, Morgan conducted a number of dihybrid crosses similar to Mendel on the Drosophila fly.

Morgan found that the dihybrid cross ratio in the F1-progeny was very different from that of G.J.Mendel’s dihybrid cross done on Pisum sativum.

Morgan attributed this to linkage and also said that genes that are physically linked to each other in chromosomes are not easily separated, so new types of combinations do not form.

According to Mendel, in his theory of inheritance, said that new types of genes combinations are formed by the separation and rearrangements of genes.

Morgan named this type of physical association of genes on chromosomes.

Later, T.H Morgan and Castel gave the chromosomal theory of linkage.

Chromosomal theory of Linkage

The main points of this theory are as follows:

1-Genes are found in chromosomes and genes are arranged in a straight sequence.

2-Genes are found on a chromosome at a special place called a locus.

3-The occurrence of linkage depends on the distance between the two genes.

4-If the distance between two genes is greater then there will be less linkage and more recombination.

5-If the distance between two genes is less then there will be more chances of linkage and less chances of recombination.

Types of linkage

There are two main types of linkage

Complete Linkage

In complete linkage, no crossing over or recombination is observed in any gene and all genes show linkage but this is very rare in nature.

However in the Drosophila fly, it was observed by Morgan.

Complete linkage is defined as a condition in which two loci with genes are so close to each other that is their alleles of these loci never separate physically by the process of crossing over or recombination.

The closer the physical location of the two genes to the DNA or the genome, the less likely they are to be separated by a crossing over that is recombination is totally absent.

For example in male fruit fly or male Drosophila, there is a complete absence of recombination process (recombinant types also absent completely) due to the absence of crossing.

This means that all genes that start with a single chromosome will end up on that chromosome in their original order. In the absence of reunion, only parental phenotype is expected.

However complete linkage is very rare in nature and not visible frequently in sexually reproducing organisms.

Incomplete Linkage

In which some genes do recombination and other genes do linkage.

It is commonly seen in nature.

Incomplete linkage produces a new set of genes in offspring due to the formation of chiasmata (‘X’ shaped structure formed during the crossing over during meiosis-I of prophase-I of Pachytene sub stage) or cross-linked genes on homologous chromosomes.

Linkage Groups

The number of haploid chromosomes present in the organism is called linkage group.

It is also the limitation of linkage. Such as –

• The total chromosome number in Drosophila is 4 pairs so the linkage group will be half that is 4.
• Similarly, the total number of chromosomes in humans is 46 and the linkage group will be half that is 23.
• The total chromosome number in the garden pea (Pisum sativum) is 14 or 7 pairs, and then the linkage group will be 7.
• The total chromosome number in Neurospora is 7 and therefore the linkage group will also be 7.
• Similarly, if the total chromosome number in Mucor fungi is two, then the linkage group will also be two.

Significance of linkage

1-Linkage does not allow the formation of a new type of gene combination, the purity of which is maintained from generation to generation.

2- This does not bring new types of gene combinations in the daughter cells, so there is no change in the traits from generation to generation.

3- Linkage reduces recombination which helps in maintaining the traits or characteristics of the crops.

4-Linkage refers to the close coexistence of genes or DNA strands on chromosomes. And these shared localizations are so close that anyone can use the genetic tools that are being passed on to families together.

And the closer the genes are to each other on the same chromosome, the better chance you have of finding the connection, because children are more likely to pass from parents to children, from grandparents, together because of meiosis.

The chances of reunion are low between these genes and thus the two genes that move together, we say they are connected to each other.

5- Genetic correlation and association analysis are great tools for identifying the genetic basis of diseases or traits.

The main difference between the two methods is that the analysis of the correlation looks at the relationship between locus and disease / trait transmission within families, while the association analysis looks at the relationship between a specific allele and disease / trait within the population.

Focuses Over the last three decades, many genetic mutations in underlying diseases or traits have been identified and these two methods can be used to detect different types.

Conclusion

From this article, we tried to understand that if linkage is happening in all the genes, then there is no crossing over process.

Do give your opinion about how you liked this article; any kind of suggestion will be welcome.

Have a great online journey!!

Thank you so much for giving your valuable time

Thank you!! Thanks!!