McClintlock & Maize Chromosome Analysis - Part II

The trisomic stock for the shortest of the ten maize chromosome segregated 2:1 for the R-r phenotype. The genetic stocks that were normal diploid(2n) for the shortest chromosome(chr.10), but were trisomic(2n+1) for, say chr. 9, segregated in a 1:1 ratio for the R-r phenotype. This proved that the extra chromosome carried the R locus when the extra chromosme was the shortest chromosome (chr.10). Cytological examination through root-tip smears showed that the genetic stock that segregated in the trisomic ratio for the R-factor had three of the shortest chromosomes.

The duplex female RRr should segregate 2R:2Rr:1RR:1r. Assuming that all female gametes are viable whether in the n or n+1 form, the phenotype ratio when crossed to a diploid testcross male should be:

Which is a 5 R_:1rr ratio. The female gametic ratio for the duplex (RRr) would be 3 n: 3 n+1. This is because one-half of the female gametes would be expected to carry the extra chromosome. McClintlock reported that only one-third of the eggs carry the extra chromosome. There is some selection against n+1 female gametes due to lagging of the extra chromosome during meiosis I and meiosis II. This was due to differences in frequency with which the n and n+1 megasporos were formed (Einset, 1943).