The segregation pattern of a translocation quadrivalent

The segregation pattern of a translocation quadrivalent was studied in three hybrid families of the tetraploid (2n=28) oat, of the Avena strigosa polyploid complex. The rate of IV formation in F₁ was high and the fertility was normal. The adjacent-alternate orientation of this quadrivalent was very variable. Conspicuous variation in this frequency was found between anthers of the same floret and between florets. The alternate type was usually more common toward the end of MI. The adjacent-alternate ratio was found to be an unreliable measure for calculating the type of gametes produced by the F₁ and the F₂ plants derived from them. An attempt was made to determine the type of viable gametes produced on the F₁ and their frequency by examining the cytology of F₂ plants. The various combinations of the chromosomes of the translocation complex expected in the F₂ plants were derived and their expected frequencies calculated by taking into account chiasma formation at the chromosome ends and various restrictions of gamete viability. In none of the three hybrid families F₂ individuals were found to produce trivalents as the most complex chromosome configuration, indicating that one type of gamete derived from adjacent separation was not formed. The 1:1 ratio between F₂ plants having only bivalents (2II) and those with F ₁-like quadrivalent configuration R(c), expected when gametes resulting from alternate separation are fully functional, with the exclusion of other types, was not found. That ratio 2II/R(c), was 2-1/3 in the various families. The cytology of selected F₃ individuals basically followed the predictions based on chromosome association in the F₂.