Difference Between Linkage and Crossing Over: Definition and Explanation

Linkage and crossing over are genetic events that occur in eukaryotic organisms during the meiosis phase of sexual reproduction. Each of the genetic events results in a different set of chromosome pairs passed to the offspring. Notably, the genetic linkage or linkage is an exception to George Mendel's Law of independent assortment. The law stated that each of the traits is inherited independently of other traits, but shortly after his works were rediscovered expectations were found. While chromosomal crossover or crossing over, was discovered by Frans Alfons Janssens in 1909, whereas the theory was established by Thomas Morgan in his book Mechanism of Mendelian Heredity (1915). Here we will know more about linkage and crossing over, their differences, consequences and go through some important questions.

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Key Takeaways: Linkage, crossing over, chromosome, gene, traits, meiosis, chromatids, sexual reproduction 

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Genetic Linkage 

The tendency of genes or DNA sequences in a chromosome to be inherited together during the meiosis phase of sexual reproduction is termed as Genetic linkage or simply linkage. This tendency was first observed by British geneticists. They tried to reproduce the results of Mendel’s work for flower color (purple and red) and the shape of pollen grains(long and round). When they crossed purebreds of purple flowers with long pollen and red flowers with round pollen. The occurrence of purple-colored flowers with long pollen was higher in offspring than expected. And similar results were found for red flowers with round pollen. But astonishingly recombinant alleles of the mentioned traits were significantly less than expected from Mendel’s Law of independent assortment. It was concluded that the genes of such traits were related because of their closeness on a chromosome. Read more on Chromosomes and Genes

Later works by Thomas Morgan and his students clarified the phenomena; it was observed that the distance between genes on a chromosome determined the frequency of recombination. As when the frequency of recombination between two genetic markers is high if the respective genes are further apart from each other, it is called unlinked genes. Whereas if two genetic markers have a low frequency of recombination then they are assumed to be proximity, which is called linked genes. And more likely to be inherited together. Thus the relative positions of two genes or associated genetic markers are described in terms of their frequency of recombination in linkage map or genetic map. 

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Crossing Over or Chromosomal Crossover

When genetic material gets exchanged between non-sister chromatids of homologous chromosomes during sexual reproduction is called crossing over or chromosomal crossover.

Often the occurrence of crossing over of genetic materials happens in matching regions of similar chromosomes. And the unlinked genetic markers are more likely to undergo such exchanges. Moreover, as genes are positioned further apart the frequency of recombination increases and hence they undergo crossover.

However, two popular theories are stating the principle of such genetic events: genetic repair theory and bacterial transformation theory. In genetic repair theory, the damaged parts or broken strands of the DNA get replaced during meiosis. Whereas, the bacterial transformation theory suggests that the meiosis evolved from bacterial transformation to propagate genetic variance. However, evidence concludes that crossing over might have evolved from bacterial transformation, which successively developed from DNA repair, thus interlinking each of the processes.

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Difference between Linkage and Crossing Over

As mentioned above the frequency of recombination between two genetic markers or genes is used to describe the likelihood of linkage and crossing over. It is worth mentioning that the chromosomal crossover can only occur during the pachytene stage of prophase I of meiosis. On the other hand, it is not so with linkage. It keeps genes together during meiosis by the virtue of their proximity. This leads us to note the differences between the two genetic events, which are as follows.

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Points to Remember

• The tendency of genes or DNA sequences in a chromosome to be inherited together during the meiosis phase of sexual reproduction is termed linkage. Whereas, when genetic material gets exchanged between non-sister chromatids of homologous chromosomes during sexual reproduction is called crossing over.
• The frequency of recombination between two genetic markers is high if the respective genes are further apart from each, it is called unlinked genes. Whereas if two genetic markers have a low frequency of recombination then they are assumed to be proximity, which is called linked genes.
• The linked genes are more likely to be inherited together during meiosis. On the other hand, unlinked genes in a chromosome are more likely to go through a chromosomal crossover. 
• The phenomenons can be associated with the distance between two genes; genes getting inherited together are inversely proportional to the distance between them in the chromosome. And genes getting segregated into different gametes is directly proportional to the distance between them in the chromosome.
• The crossing over occurs for DNA repair, which developed in bacterial transformation, to produce genetic variation. Whereas through genetic linkage the progenies carry forward the parental lineage of taits. 

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