What is the relationship between the degree of crossing over and the distance between two genes?

The main difference between linkage and crossing over is that the linkage ensures that genes in the same chromosome are inherited together whereas the crossing over ensures the separation of genes in the same chromosome, segregating them to different gametes. 

Linkage and crossing over are two mechanisms in genetics that influence genes in the same chromosome. Here, the distance between two genes on the same chromosome is inversely proportional to the degree of linkage while the distance between two genes on the same chromosome is directly proportional to the chance of crossing over.

Key Areas Covered

1. What is Linkage
     – Definition, Facts, Importance
2. What is Crossing Over
     – Definition, Facts, Importance
3. What are the Similarities Between Linkage and Crossing Over
     – Outline of Common features
4. What is the Difference Between Linkage and Crossing Over
     – Comparison of Key Differences

Key Terms

Crossing Over, Genes in the Same Chromosome, Genetic Variation, Linkage, Recombination

Linkage is the tendency of genes to stay together in the same chromosome. The genes in a particular chromosome that are inherited together are called a linkage group, which is equal to a chromosome. Due to the linkage, the genes in the same chromosome will be inherited together during the formation of gametes.

Figure 1: Gene Linkage Map of Drosophila

The genes located in very close proximity on the same chromosome are completely linked. These genes are never subject to recombination. On the other hand, genes located far apart on the same chromosome are incompletely/partially linked. Therefore, they can be easily separated during recombination. But, genes in different chromosomes are not linked, and the independent assortment of these genes can be described in the dihybrid cross.

What is Crossing Over

Crossing over is the exchange of genetic material of the non-sister chromatids of homologous chromosomes. So, it can be described as the tendency of genes in the same chromosome to separate from each other and segregate into different gametes. Hence, crossing over facilitates the genetic recombination to produce new combinations of alleles.

Furthermore, crossing over occurs at the synapse, the tight holding that keeps the two homologous chromosomes together. It takes place during the pachytene stage of the prophase 1 of meiosis. The main advantage of crossing over is that it allows the origin of new characters in the offspring, which leads to genetic variation.

Definition

Linkage refers to the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction while crossing over refers to the exchange of genes between two chromosomes, resulting in non-identical chromatids that comprise the genetic material of gametes.

Significance

Linkage ensures that genes in the chromosome are inherited together while crossing over ensures the separation of genes on the same chromosome to segregate into different chromosomes. This is the main difference between linkage and crossing over.

Distance Between Two Genes

Another difference between linkage and crossing over is that the distance between two genes is inversely proportional to the degree of linkage while the distance between two genes is directly proportional to the chance of crossing over.

Maintenance of the Parental Traits

Also, linkage ensures the maintenance of the parental traits in the offspring while crossing over causes changes in the parental trait.

Influence of the Age

Another difference between linkage and crossing over is that the linkage decreases with age while the crossing over increases with age.

Variations

Furthermore, the linkage reduces the chance to produce variations in the offspring while crossing over increases the chance to produce variations in the offspring.

Conclusion

Linkage is the tendency of genes to inherit together in the same chromosome. The distance of the genes is inversely proportional to the degree of linkage. On the other hand, crossing over is the tendency of genes in the same chromosome to separate into different gametes. It leads to the genetic recombination, producing genetic variation. The main difference between linkage and crossing over is the influence of each mechanism on the genes in the same chromosome.  

Reference:

1. “Genetic Linkage.” Learn Genetics, University of Utah, Available Here
2. Cornell, Brent. “Crossing Over.” BioNinja, 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  

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