9.4 Coupling and Repulsion (cis and trans) Configuration
Just by looking at an organism that is heterozygous at two loci, you cannot tell how the mutant and wild type alleles are arranged. Both mutant alleles could be on one homologous chromosome, and both wild type alleles could be on the other (e.g., a–b– / A+B+). This is known as a coupling (or cis) configuration. When one wild type allele and one mutant allele are on one homologous chromosome, and the opposite is on the other, this is known as a repulsion (or trans) configuration (e.g., A+b– / a–B+). The way to determine the orientation is to look at the parents (or P generation) of that cross if you know the genotypes of them. If the parents are homozygous for both genes, and one shows both dominant phenotypes and the other shows both recessive phenotypes, then you know that the individual you are looking at is in coupling configuration. If one parent has one dominant and one recessive phenotype, and the other has the opposite, then you know the individual is in repulsion configuration.
The following video, Genetics! coupling (cis) vs Repulsion (trans), by Medaphysics Repository (2015) on YouTube, discusses the difference between cis and trans genes.
The video, Coupling vs Repulsion, by Genetics Rocks (2019) on YouTube, looks at a worked example involving observed frequencies in a text cross and genes in coupling/repulsion.
- Figure 9.4.1 Original by Deyholos (2017), CC BY-NC 3.0, Open Genetics Lectures
Deyholos, M. (2017). Figure 5. Alleles in coupling configuration…[digital image]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 18, p. 4). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf
Genetics Rocks. (2019, September 14). Coupling vs repulsion (video file). YouTube. https://www.youtube.com/watch?v=llNZP1Wmgok
Medaphysics Repository. (2015, February 24). Genetics! coupling (cis) vs Repulsion (trans) (video file). YouTube. https://www.youtube.com/watch?v=4y5vjhMq6iY
- Figure 9.4.1 Two cells showing alleles in either the coupling or cis configuration (whereby both mutant alleles are present on one homologous chromosome, and both wild type alleles are present on the other) or the repulsion or the trans configuration (whereby one wild type allele and one mutant allele are on one homologous chromosome, and the opposite is on the other). [Back to Figure 9.4.1]