Conclusion

The MP element was later shown to be identical to McClintock's Ac element. The MP element was inserted into the RR allele to create the RRMP complex allele. RRMP produced variegated pericarp and the RR allele produced red pericarp. Therefore the MP element influenced expression of the RR allele. The RRMP complex was somewhat unstable and the MP element could transpose away from RR such that RRMP became RR + MP. RR + MP gave red pericarp.

RR + MP RRMP RR + MP
red pericarp variegated pericarp red pericarp

The MP element could transpose to either the same chromosome as the RR allele or a non-homologous chromosome. Sometimes MP was linked to RRMP and resulted in light variegated pericarp. Otherwise MP might not be associated with RRMP, depending on where this second Mp element was transposed. When a second Mp element was not linked to the RRMP complex, the light variegated pericarp was still expressed.

The ratio of medium to light variegated pericarp depended on the linkage relationship between the second Mp element and RRMP.

Brink (1958. Genetics 43:435-447) later showed that MP (Ac) could simultaneously cause chromosome breakage where the Ds element was located and also alter expression of the RR allele. The Ac and Ds elements can transpose to various locations in the genome. This was shown with linkage to morphological markers. The Ac element inserted into the waxy allele (Wx), can interrupt gene expression and convert the dominant allele to a recessive mutant. Later the Ac element could leave the Wx allele, reversing the allele back to Wx.

Wx + Ac WxAc Wx + Ac
normal    waxy mutant   normal
starch waxy starch starch

How can a geneticist recognize that the Ac element is inserted in a gene? Because Ac creates an unstable mutation that can mutate back and because Ac can also cause dissociation at site of Ds.