5. Frequency of transmission of the extra chromosome in trisomes. Different trisomic stocks derived as maternal 21 chromosome plants from tetraploids show decided differences in percentage of trisomic plants in the progenies. Marked differences have also been observed in univalent frequencies, frequency of lagging in anaphase I and II and in other details of meiosis. A stock in which 40% of the progeny was found to be trisomic had one of the longer chromosomes in triplicate. Another stock producing 24% trisomic progeny had one of the shorter chromosomes in triplicate.
In order to test whether length of the extra chromosome can be correlated with frequency of transmission, known stocks have been studied. The data presented are incomplete but may be of some interest.
As the table indicates, the frequency of transmission of the extra chromosome through the egg varies from 22% to 52%. Different stocks of the same trisome show considerable variability in frequency of 2n + 1 progeny. However, there is a strong positive correlation between length of the extra chromosome and the frequency with which it is transmitted through the egg. Several of the cases which are out of line may be due to the small number of seeds available.
Such explanations as abortion of ovules or differential seed viability would not seem to account for the observed differences in frequency of transmission since a close correspondence is found between the percentage of progeny which is 2n+1 and the percentage of microspores with the n+1 number (see table 2).
Sporocyte studies, which have not yet been completed, indicate a greater frequency of univalents in the shorter chromosome stocks with more lagging in Met. I. and the formation of a greater number of micronuclei.