Difference Between Coupling and Repulsion

The key difference between coupling and repulsion is that coupling refers to the linkage of two dominant or two recessive alleles while repulsion refers to the linkage of dominant alleles with recessive alleles.

Based on his experiments, Gregor Mendel stated three laws of inheritance: the law of dominance, the law of segregation and the law of independent assortment. Law of independent assortment states that genes for different traits segregate independently of each other during the gamete formation. However, some experiments show failure in the independent assortment of genes. During the test crosses, the expected ratios are not observed. This is due to genetic linkage. Coupling and repulsion are two different aspects of linkage. Coupling refers to the linkage of two dominant or two recessive alleles while repulsion refers to the linkage of dominant alleles with recessive alleles.

CONTENTS

1. Overview and Key Difference
2. What is Coupling 
3. What is Repulsion
4. Similarities Between Coupling and Repulsion
5. Side by Side Comparison – Coupling vs Repulsion in Tabular Form
6. Summary

What is Coupling?

Coupling is the linkage of two dominant alleles of two genes in one chromosome and two recessive alleles of two genes in the other homologous chromosome. Here, dominant alleles of the genes are present in one chromosome, while their recessive alleles are present in the other chromosome. These linked genes show cis arrangement. It can be illustrated as AB/ab.

One chromosome carries AB and the other carries ab. This physical coupling between dominant alleles and between recessive alleles prevents their independent assortment during the gamete formation. Dominant alleles tend to remain together. Similarly, recessive alleles also tend to remain together during the gamete formation.

What is Repulsion?

Repulsion is another aspect of linkage which is different from coupling. In repulsion, dominant alleles or recessive alleles come from different parents, and they tend to remain separate. Here, one parental chromosome carries one dominant and one recessive allele while the other chromosome carries the other two alleles (dominant and recessive alleles). It can be illustrated as Ab/aB.

One dominant allele is linked with the recessive allele of the second gene. This type of gene arrangement is called trans arrangement.

What are the Similarities Between Coupling and Repulsion?

• Coupling and Repulsion are two aspects of linkage.
• They behave against Mendel’s law of independent assortment.

What is the Difference Between Coupling and Repulsion?

In coupling, there is a tendency in dominant alleles to remain together; there is a tendency in recessive alleles to remain together. On the other hand, in repulsion, two such dominant alleles or two recessive alleles come from different parents, and they tend to remain separate. So, this is the key difference between coupling and repulsion. The observed ratio in coupling is 7:1:1:7 while the ratio of repulsion is 1:7:7:1. Moreover, coupling is a type of cis arrangement, while repulsion is a type of trans arrangement.

The following infographic shows more comparisons related to the difference between coupling and repulsion in tabular form.

Summary – Coupling vs Repulsion

Coupling and repulsion are two aspects of gene linkages. Coupling is the presence of two dominant alleles of two genes on the same chromosome (AB). The remaining recessive genes of the two genes are present on the other chromosome (ab). Hence, the dominant alleles of genes tend to remain together. Repulsion is the presence of dominant genes on the two homologous chromosomes (Ab/aB). Therefore, dominant alleles or recessive alleles that came from different parents tend to remain separate. Thus, this is the summary of the difference between coupling and repulsion.

Reference:

1. Griffiths, Anthony et al. “The Discovery Of Linkage”. Ncbi.Nlm.Nih.Gov, 2020, Available here.
2. “Coupling And Repulsion Hypothesis Of Gene”. Biology Discussion, 2020, Available here.