2.3 Two Loci on One Chromosome

Based on the description in the last section, it would be expected that if the genes were on the same chromosome the alleles would travel together through meiosis (Figure 2.3.1 top). However, when tested this is not always the case. The recombination of alleles can be explained through the phenomenon of crossing over, which occurs during prophase I.

Crossing over is an exchange between non-sister chromatids that can occur at any position along the entire chromosome. If the two loci that are being considered are sufficiently separated from each other on the chromosome, crossover events can occur between the two loci.

This coupled with the random orientation that the chromosomes align during metaphase I, will allow the other combination of alleles in the gametes (Figure 2.3.1 bottom).

While not shown in Figure 2.3.1, if the two loci are very far apart, multiple crossover events can also take place, further increasing the shuffling of alleles.

Independent assortment as seen on the same chromosome, coloured green
Figure 2.3.1 Independent Assortment as Seen on the Same Chromosome. On the top is an example of what would happen if crossovers do not occur. The dominant alleles of gene A and gene D would travel together, not leading to independent assortment. Crossovers do occur in most situations though, like in the bottom half of the figure. If a crossover occurs between the two genes, then the alleles will transfer to the other non-sister chromatid, thus rearranging alleles. This allows for independent assortment, despite being on the same chromosome. This is just one of the many arrangements or crossover events that could occur during meiosis, with every meiocyte arranging themselves differently with different crossovers.

The farther apart on the chromosome the more crossover events (Figure 2.3.2) will take place between the two loci. Ultimately, this will result in similar allele combinations to those observed in independent assortment shown above, even if they are on the same chromosome.

Blue and red homologous chromosomes demonstrating crossing over
Figure 2.3.2 Crossover of Genetic Material During Prophase I of Meiosis

If the loci are very close together on the same chromosome, fewer crossovers are likely occur between them. We will not discuss this situation in here, but will do so later on.

Media Attributions

References

Canham, L. (2017). Figure 4. Independent assortment as seen on the same chromosome. [digital image]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 17, p. 2). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf

Rye, C., Wise, R., Jurukovski, V.,DeSaix, J., Choi, J., & Avissar, Y. (2016, October 21).  Figure 11.3 Crossover occurs between non-sister chromatids of homologous chromosomes [digital image]. CNX OpenStax Biology (Chapter 11). https://openstax.org/books/biology/pages/11-1-the-process-of-meiosis

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Open Genetics by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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