Study and use of trisomics.
1. The frequency of transmission of trisomics without root‑tip chromosome counts can be determined by crossing each trisomic with a homozygous translocation involving that chromosome. The trisomic F1 plants will show low pollen sterility (25‑30 per cent) as compared with the 50 per cent shown by their diploid sibs. With experience the difference can be recognized easily even in the field with the "pocket microscope". I have used it satisfactorily for chromosome 6, using T 5‑6a.
2. It would also be desirable to make the trisonic analysis usable by those not able to get chromosome numbers counted. At present only plants trisomic for chromosomes 5 and 7 are phenotypically distinguishable in most crosses, but not in all.
Two tertiary trisomic stocks for each chromosome might be established so that between them the entire chromosome in question rould be represented in trisomic condition. If the piece of the attached non‑homologue which is also trisomic came from chromosome 5 or 7, it might serve to identify the desired tertiary trisomic plants. Since these tertiaries would, also differ from primary trisomics by having approximately 15 per cent of pollen abortion while the primaries would be normal, pollen examination could be used as a supplementary check if desired or if the phenotypes were not distinct.
In place of the 10 primary trisomics, 20 tertiary types would be used for a complete test of the 10 chromosome or linkage groups.
For example, the series might be established from 2n + 1 (No. 1 chromosome trisomic) x T 1‑5; 2n++ 1 (No, 2 chromosome trisomic) x T 2‑5, etc., selecting the translocation in each case in which the break in 5 was near the middle of the chromosorte, assuming a plant trisomic for nearly half of 5 would be most likely to be phenotypically distinct. Two tertiaries would be established for each cross. A series with chromosome 7 also might be usable.
C. R. Burnham