What is the Difference Between Linked and Unlinked Genes

The main difference between linked and unlinked genes is that the linked genes sit close together in the chromosome whereas the unlinked genes sit farther away from each other in the chromosome. Furthermore, linked genes have a chance to be inherited together while unlinked genes are more likely to separate during the formation of gametes in a process known as homologous recombination.

Linked and unlinked genes are the two types of genes in the genome. Their patterns of inheritance cause genetic variation.

Key Areas Covered

1. What are Linked Genes
     – Definition, Location, Inheritance
2. What are Unlinked Genes
     – Definition, Location, Inheritance
3. What are the Similarities Between Linked and Unlinked Genes
     – Outline of Common Features
4. What is the Difference Between Linked and Unlinked Genes
     – Comparison of Key Differences

Key Terms

Chromosomes, Homologous Recombination, Inheritance, Linked Genes, Unlinked Genes

What are Linked Genes

Linked genes are the genes that are close to each other in the same chromosome. Due to their close proximity, they do not show independent assortment as mentioned in Mendel’s second law. Therefore, linked genes are more prone to be inherited together during the formation of gametes. The reason behind this type of inheritance is the inability of these genes to undergo homologous recombination.

Figure 1: Drosophila Linkage Map

Based on the patterns of inheritance of genes, scientists can create maps of genes, revealing the relative distances between genes. Hence, this process is called gene mapping. If two genes are inherited together more than the 50% of the time, then these two genes are linked genes.

What are Unlinked Genes

Unlinked genes are the genes that are farther away from each other in the same chromosome or in different chromosomes. These genes separate independently from each other during the formation of gametes. This is due to the ability of these genes to undergo homologous recombination.

Figure 2: Homologous Recombination

Generally, a single chromosome contains thousands of genes and some gene pairs are close to each other while the distance between the other gene pairs may be high. In addition, most organisms are diploid and they contain two chromosomes with the same arrangement of genes, the homologous chromosomes. During the formation of gametes, DNA replicates and the centromere holds the sister chromatids of each chromosome. When the sister chromatids of the homologous chromosomes align together during the metaphase, the DNA strands of them can break and rejoin, swapping large parts of the chromosomes. Then, these rearranged homologous chromosomes separate into gametes. Thus, in order to be separated by homologous recombination, a particular gene pair should be in a considerable distance in the chromosome. However, the chance of unlinked genes to be inherited together is 50%.

Similarities Between Linked and Unlinked Genes

• Linked and unlinked genes are the two types of genes in the genome.
• Both inherit to the gametes during the sexual reproduction.
• Also, both their patterns of inheritance cause genetic variation.
• Furthermore, a test cross can reveal the degree of linkage.

Difference Between Linked and Unlinked Genes

Definition

Linked genes refer to the genes that are inherited together with the other gene(s) as they are located on the same chromosome while unlinked genes refer to the genes located farther apart from each other. Hence, this explains the fundamental difference between linked and unlinked genes.

Chromosome

Also, another difference between linked and unlinked genes is that the linked genes occur in the same chromosome while the unlinked genes occur either in the same chromosome or in different chromosomes.

Distance between Genes

Linked genes sit closely together while unlinked genes sit farther apart. This is the main difference between linked and unlinked genes.

Homologous Recombination

Furthermore, linked genes do not undergo homologous recombination while unlinked genes undergo homologous recombination.

Inheritance

Linked genes are inherited together while unlinked genes have a less chance to be inherited together. This is also a difference between linked and unlinked genes.

Chance to Inherit together

Moreover, in linked genes, the chance of inheriting together is more than 50%. In contrast, in unlinked genes, the chance of inheriting together is less than or equal 50%.

Mendel’s Second Law

Another important difference between linked and unlinked genes is that linked genes do not follow Mendel’s second law while unlinked genes follow Mendel’s second law.

Dihybrid Ratio

Besides, the dihybrid ratio of linked genes is 3:1 while the dihybrid ratio of unlinked genes is 9:3:3:1.

Test Cross Ratio

In addition, the test cross ratio of linked in genes in a dihybrid cross is 1:1 while the test cross ratio of unlinked genes in a dihybrid cross is 1:1:1:1.

Conclusion

Linked genes occur in close proximity on a particular chromosome. Therefore, they are unable to undergo homologous recombination. Hence, linked genes have a high chance to be inherited together. On the other hand, unlinked genes occur farther apart in a particular chromosome or in different chromosomes. Therefore, they have a higher chance to undergo homologous recombination, which allows the separation of the unlinked genes to different gametes. Thus, only unlinked genes follow Mendel’s second law. They can cause genetic variation through homologous recombination. Therefore, the main difference between linked and unlinked genes is the distance and the patterns of inheritance.

Reference:

1. “Genetic Linkage.” Learn.Genetics, University of Utah, 2 Dec. 2014, Available Here

Image Courtesy:

1. “Drosophila Gene Linkage Map” By Twaanders17 – Own work (CC BY-SA 4.0) via Commons Wikimedia  
2. “Figure 12 03 04” By CNX OpenStax –  (CC BY 4.0) via Commons Wikimedia