In coupling, dominant alleles are present on the same chromosome and recessive alleles are present on the same chromosome. In repulsion, dominant alleles of some genes and recessive alleles of some genes are present on the same chromosome. 

Coupling and Repulsion are two faces of crossing over and linkage as when two genes are present on the same chromosome, they are linked and so, crossing over cannot occur.

Coupling and Repulsion in Linkage

Coupling and repulsion are the two different types of gene arrangements that occur in linkage, which is the phenomenon where genes located on the same chromosome tend to be inherited together.

Coupling

Coupling refers to the arrangement where the dominant and recessive alleles of two different genes are present on the same chromosome. This means that the parental chromosome contains the dominant allele of one gene and the recessive allele of another gene, while the other chromosome contains the recessive allele of the first gene and the dominant allele of the second gene.

For example, consider two genes A and B located on the same chromosome. If the parental chromosome has the dominant allele of A and the recessive allele of B, it is said to be in coupling. The other chromosome in this case will have the recessive allele of A and the dominant allele of B.

Repulsion

Repulsion refers to the arrangement where each chromosome contains one dominant and one recessive allele. This means that the parental chromosome contains the dominant allele of one gene and the recessive allele of the other gene, while the other chromosome contains the recessive allele of the first gene and the dominant allele of the second gene.

For example, consider two genes C and D located on the same chromosome. If the parental chromosome has the dominant allele of C and the recessive allele of D, it is said to be in repulsion. The other chromosome in this case will have the recessive allele of C and the dominant allele of D.

Significance

The coupling and repulsion of genes in linkage have important implications in genetics. If two genes are in coupling, they tend to be inherited together and are said to be in linkage disequilibrium. This means that the frequency of the parental and recombinant gametes is not equal, which results in non-Mendelian ratios of progeny. On the other hand, if two genes are in repulsion, they tend to be inherited independently and are said to be in linkage equilibrium. This means that the frequency of the parental and recombinant gametes is equal, which results in Mendelian ratios of progeny.