The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations. III. Telocentric trisomics

The estimation of the crossing-over potentials of the two arms of a specific chromosome that can not be recognized in the diploid, was earlier found to be inefficient with the use of the primary trisomic. With the telocentric trisomics two groups of two different configurations each can be recognized that permit a reasonably exact estimation of the two parameters. Each telocentric trisomic yields estimates for both arms. The trisomic arm is underestimated as a result of partner-exchange and/or interference by the nucleolus in a nucleolus bearing arm. The other arm is estimated more correctly. Thus the two telocentrics together give a complete picture of the chromosome. After a correction for differences in overall chiasma frequencies the ratio of the crossing-over potentials of the two arms of the satellite chromosome ofSecale cereale was found to be approximately 2. This is large for a submedian chromosome in comparison with the ratio for the genome as a whole and it is attributed tentatively to the nucleolus interfering with chiasma formation in the short arm. It is suggested that the three homologous arms, especially the long arms, differ in respect to the tendency to pair and in chiasma frequency.