Repulsion and coupling are two faces of

The correct option is linkage.

To understand why repulsion and coupling are two different phases of linkage, let us look at the concept of genetic linkage and how genes on the same chromosome behave during inheritance.

Linkage refers to the physical association of genes on the same chromosome. Genes that are close to each other on the same chromosome tend to be inherited together because they do not assort independently during meiosis. Thomas Hunt Morgan and William Bateson, Reginald Punnett, and others observed that linkage can present itself in two distinct arrangements, which are referred to as phases: the coupling phase and the repulsion phase.

1. Coupling Phase (cis-configuration):
In the coupling phase, dominant alleles of two or more linked genes are present on one chromosome, and their corresponding recessive alleles are present on the homologous chromosome. For example, if we have two genes A and B, the arrangement on the homologous chromosomes is:
Homologue 1: A B
Homologue 2: a b
Here, the dominant alleles (A and B) are linked together ("coupled") on one chromosome, while the recessive alleles (a and b) are linked on the other.

2. Repulsion Phase (trans-configuration):
In the repulsion phase, a dominant allele of one gene and a recessive allele of the other gene are present on the same chromosome. The homologous chromosome carries the opposite combination (recessive allele of the first gene and dominant allele of the second). For the same genes A and B, this arrangement is:
Homologue 1: A b
Homologue 2: a B
Here, the dominant allele of one gene is linked with the recessive allele of the other, meaning the dominant alleles A and B are on different chromosomes and appear to "repel" each other.

Since both coupling and repulsion describe the specific spatial arrangements of linked genes on homologous chromosomes, they are referred to as the two faces or phases of linkage.