3. A tentative hypothesis concerning the genetic basis of the difference between light variegated and medium variegated pericarp.
The parallel nature of the results obtained on testing the VV and WR segregates from the two classes of VV/WR F1 plants, light and medium variegated, in sections 1 and 2 above, is evident. The data summarized in section 2 show that the genetic basis of the difference between light and medium variegated is readily separable from the VV locus itself. The critical fact is that the differential element, whatever it may prove to be, assorts with WR gametes as well as with VV gametes. Furthermore, the frequencies with which light and medium variegated plant, appear among the respective VV and appropriately tested WR segregates from F1 VV/WR plants of corresponding phenotype are parallel and apparently of similar magnitude.
The data already available reveal possibly important deviations in a few families from the pattern of inheritance which otherwise prevails. These apparent exceptions have not been included in the report, and they cannot be explained at present without recourse to wholly untested assumptions. Moreover, the omission of the red pericarp F1 plants from the tests of the WR segregates leaves a gap in the evidence which it is essential to fill, especially in view of the known fact that the change fram variegated to red involves mutation at the P locus. My hypothesis advanced on the basis of the current evidence is to be considered tentative, therefore, any may call for more or less radical modification as additional facts are established.
It is assumed that a "modulator" locus distinct from the P locus, and probably remote from it, governs the level of phenotypic expression of variegated pericarp in these stocks. If a particular modulator allele, Mp1, is present in a variegated plant the pericarp is light variegated; if mp2 is substituted for Mp1 the pericarp is medium variegated Mp1 aud mp2 mutate to each other in somatic tissue with relatively high frequency. The inbred WR lines used carry still other mp alleles, recessive to Mp1, whose phenotypic effects on variegation are like those of mp2. The modulator alleles in certain of the inbred lines appear to be stable; those in other lines possibly may be mutable since irregular ratios of light and medium variegated plants were observed in a few families.
On these assumptions: (1) the foundation plant, S1937‑10, was a (mp2/mp2) VV/VV – Mp1/mp2 VV/VV ‑‑ ?/? RR/VV) chimera and (2) the ratios in which light variegated and medium variegated plants appeared in the several families descended from S1937‑10 are determined by the assortment of the particular modulator alleles present including those newly arisen by mutation.
Mutation of variegated to red at the P locus is related to the alleles at the modulator locus. This conclusion is in accord with the well recognized fact that the phenotypic differences between light variegated and medium variegated plants rests on a dissimilarity in frequency and distribution during development of VV to RR mutations at the P locus.