In genetics, coupling and repulsion are the two states of linkage. Linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together, rather than assorting independently during meiosis.
In the coupling state, the two alleles of different genes that are being tested are present on the same chromosome, while in the repulsion state, the two alleles are present on different chromosomes. The coupling state increases the likelihood of the two alleles being inherited together, while the repulsion state increases the likelihood of the two alleles being inherited separately.
Chiasma, mutation, and crossing over are not states of linkage. Chiasma refers to the physical exchange of genetic material during meiosis, while mutation refers to a change in the DNA sequence. Crossing over refers to the exchange of genetic material between homologous chromosomes during meiosis.