Observations upon chromosome associations

While observations upon chromosome structure, movements and associations in normal diploids have provided us with fundamental conceptions of the role played by the chromosome in Mendelian heredity, it has been clear for some time that further progress is to be sought in the examination of abnormal or unusual types. In recent years, the chief contributions to an understanding of the conditions underlying chromosome associations have resulted from studies upon organisms whose chromosomes show numerical or structural aberrations from the normal. The abnormalities may arise de novo, but most significant results are likely to be obtained when they are induced by such agencies as X-rays in experimental material. At present it appears that exceptional types of pairing, such as the non-homologous pairing observed by McClintock and others, or the anomalous chromosome association at metaphase in Drosophila pseudo-obscura, are also of particular significance in relation to the whole problem of chromosome associations. In addition to a general consideration of chromosome associations, the present study records observations upon chromosome pairing in Avena hybrids, on the pairing and segregation of heteromorphic homologous chromosomes in Triticum, and on the anomalous pairing of the m-chromosomes of Anasa tristis