McClintlock & Maize Chromosome Analysis - Part I

See - McClintlock, B, and H. E. Hill. 1931. The cytological identification of the chromosome associated with the R-G linkage group in Zea Mays. Genetics 16:175-190

See - Einset, J. 1943. Chromosome length in relation to transmission frequency of maize trisomes. Genetics 28:249-364.

McClintlock used trisomic analysis to determine which of the ten maize chromosomes was associated with a particular linkage group. She discovered that that R-G linkage group was on the shortest maize chromosome. How did she prove this fact?

First she developed a set of 2n+1 or trisomics from a triploid plant. She had a primary trisomic for each of the ten maize chromosomes. trisomic for each of the ten maize chromozomes. She obtained the original trisomic plants from crossing a triploid x diploid and this resulting progeny consisted of various trisomics

The basis for trisomic analysis is that n+1 pollen is largely inviable because it is less competitive than normal haploid gametes(n). The n+1 eggs are more viable than the n+1 pollen.

The R factor for red aleurone is a dominant marker allele used to identify the segregation ratios. A primary trisomic for the chromosome which carries the R factor will have a different segregation ratio than any other trisomic chromosome which does not carry the R factor. Consider the R factor in duplex for the trisomic with this locus. The male gametic ratio is based on the concept that n+1 pollen is not viable. An RRr (duplex) male will produce pollen in the ratio:

2R:2Rr:1RR:1r

Where the Rr and RR pollen are n+1 types due to the trisomic chromosome. However, the Rr and RR are not viable. Then the above ratio reduces to:

2R:1r

Which indicates that for balanced gametes (n) we would expect two pollen which carry the R alleleomorph for each pollen that carries the r alleleomorphic form. Alternatively, consider the ratio for pollen which carry the R-r factor on non-trisomic stock. A heterozygous Rr plant would result in a 1R:1r gamete ratio.

A trisomic male heterozygous duplex for the R factor (RRr) crossed to a diploid female tester would result in the following progeny

versus a trisomic stock where the trisomic chromosome does not carry the R-factor.