Linkage tests of golden-2

Golden-2 (g2) has a more extreme phenotype than g. The culms and leaf sheaths are nearly white. The current linkage maps indicate that g2 is located on chromosome 7, locus unknown. In scoring some F2 families segregating for this trait (source: Coop. 64-169) and various chromosome 7 markers, the ease with which multiple gene combinations were recovered suggested that g2 might not be on this chromosome. A definitive answer was sought from backcross tests with sizeable populations. The first four lines of the following table summarize the relevant two point tests from the testcross g2/o2 v5 ra gl x g2 o2 v5 ra gl (3 families, 1050 total plants). The last line is from the testcross g2/bd x g2 bd (4 families, 1180 total plants). There was good homogeneity among the families of each cross for single gene segregations as well as for independence. The single factor segregations of g2 and of each of the marker genes did not deviate significantly from the expected 1:1.
Tester Mutant + + + t g2 + g2 t Total Percent recombination
o2 285 248 270 247 1050 49.3
v5 275 258 265 252 1050 49.8
ra 271 262 268 249 1050 50.5
gl 270 263 265 252 1050 50.3
bd 307 296 295 282 1180 50.1

None of these deviate significantly from independence. The known map of chromosome 7 is presently 112 crossover units long. Based on chiasma frequency the theoretical length of this chromosome is 123 map units (Rhoades, 1955), therefore the above data adequately test the entire length of this chromosome. It is concluded that g2 does not lie on chromosome 7.

This conclusion is supported by a subsequent test with an A-B translocation. When F1 plants of the constitution g2/o2 v5 ra gl were pollinated by plants carrying TB-7Lb, none of the resulting hypoploid plants were g2.

R. H. Whalen

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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