What is the exact concept of "Repulsion" and "Coupling" (Genetics)?

Question

From all sorts of books including standard books (like those by Tamarin, Griffith etc) as well as internet; I could not clear-up many things about "Repulsion" and "Coupling". Such as:-

• Is "Coupling" an exact synonym of "Linkage" ? and is "repulsion" an exact synonym of "recombinant"? No-where I could yet found mention of it.

• What are they telling as coupling-"phase" and repulsion-"phase"? Are there 2 different "step"s or "time-frames" or "generations"?

• Coupling and repulsion- are they indicating to a "phenomenon"? or the Genotypic "state"?

• according to Griffith's introduction to genetic analysis (7th Ed)

Now let us consider what may be learned by repeating the crossing experiments but changing the combinations of alleles contributed as gametes by the homozygous parents in the first cross. In this cross, each parent was homozygous for one dominant allele and for one recessive allele. Again F1 females were testcrossed:

The following progeny were obtained from the testcross:

Again, these results are not even close to a 1:1:1:1 Mendelian ratio. Now, however, the largest classes are those that have one dominant allele or the other rather than, as before, two dominant alleles or two recessives. But notice that once again the allelic combinations that were originally contributed to the F1 by the parental flies provide the most frequent classes in the testcross progeny. In the early work on coupling, Bateson and Punnett coined the term repulsion to describe this situation, because it seemed to them that, in this case, the nonallelic dominant alleles “repelled” each other—the opposite of the situation in coupling, where the dominant alleles seemed to “stick together.”

From this paragraph; it strongly seems to me; repulsion is not opposite word of coupling; rather it is synonym of coupling; where we started the breeding experiment with such parents those contained Dominant allele on one locus and recessive allele in the other locus for all the chromosomes. As a result; parental combination Pr⁺ and vg trended to stay together "coupled" (965 progenies at testcross output) and similarly parental combination pr and Vg⁺ stayed "coupled". Have I understood it correctly? or I'm missing certain significance of the concept of repulsion?

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Answer 1

I guess the confusion is stemming from trying to fit the phenotypic terms coupling and repulsion to their genotypic basis.

Coupling and Repulsion are both used considering the phenotypic profile of the progeny, and hence, takes into consideration only the distribution of the dominant allele. If, in the progeny of the test cross, the percentage of double dominant is less than that the average percentage of single dominant, that would be what we call repulsion, as in the two dominant traits tend to be separated. The opposite, percentage of double dominant being greater than the average of individual dominant, is coupling.

This said, it is clear what would be the corresponding genotypic states. If the chromosomal states are $Pr^+/Vg^+.. pr/vg$, coupling would be observed if the $pr$ and the $vg$ loci are linked, since for individual dominant phenotypes to appear, recombination is necessary which is rendered less probable due to linkage. A genotype of $Pr^+/vg..pr/Vg^+$ would result in the phenomenon of repulsion in a test cross. The case you cite, has the latter, and hence, we observe repulsion.

Now to your questions.

Is "Coupling" an exact synonym of "Linkage" ? and is "repulsion" an exact synonym of "recombinant"? No-where I could yet found mention of it.

Neither. More accurate would be to consider both coupling and repulsion as consequences of linkage, differing in the configuration of the linked gene (see above).

What are they telling as coupling-"phase" and repulsion-"phase"? Are there 2 different "step"s or "time-frames" or "generations"?

Although this is difficult to answer without knowing the context, I belive that the author here might be referring to the variable possible configuration of the alleles, as in, if they are in a coupling-phase, they are so oriented that the resultant progeny will show the coupling phenomenon. The phase may change to repulsion if a recombination event occurs. (again, see above.)

Coupling and repulsion- are they indicating to a "phenomenon"? or the Genotypic "state"?

Not to argue much in this semantic mashup, they are best viewed as differing phenomenon (as in the different obtained progeny phenotypic ratios) resulting from varied genotypic states.