An Interpretation of the Observed Amount

N an individual heterozygous for an interchange between two non-homoloI gous chromosomes, the observed amount of gene recombination in an interstitial region depends on the frequency of the respective centromere disjunction types and upon the chiasma (crossover) frequency expected for the two interstitial regions of the interchange complex. An interstitial region is considered to be the chromosome segment between the centromere and the point of interchange. LAMM (1948) presented the formulas from TOMETORP'S unpublished analysis to express this dependency of the recombination measurement upon these factors ; however, only the chiasma frequency in the interstitial region being studied was considered. The amount of gametic sterility expressed hy an interchange heterozygote as measured by the frequency of spore abortion also is a function of the frequency of centromere disjunction types and the chiasma frequencies in the two interstitial regions. BURNHAM (1948) recognized this dependency and noted that a species could have primarily alternate segregation yet exhibit sterility of 50 percent. HANSON and KRAMER (1949) have presented and discussed the relationships for the observed amount of recombination and the amount of sterility in terms of the centromere orientation frequencies and the chiasma frequencies in both interstitial regions. Based on the formulas presented, the observed recombination and the amount of gametic sterility were interpreted, and the interrelationship between these two factors was discussed. In the analysis presented by HANSON and KRAMER ( 1949), the assumption was made that at the first division only the theoretically possible 2-2 segregations from the chromosomes of the interchange complex occur, with the segregation types being independent of crossing over. In this paper these segregation types will be referred to as alternate, adjacent 1, and adjacent 2 ( MCCLINTOCK 1945) which correspond to the centromere disjunction types A, B and C, respectively, used in the 1949 analysis. Altternate and adjacent 1 segregations involve homologous centromere disjunction at division I and are differentiated on the condition that alternate or adjacent chromosomes pass to the same pole. Adjacent 2 segregation involves nondisjunction of homologous centromeres and disjunction of the translocated pieces. Also, the assumption was made that only single crossing over occurs within an interstitial region, but that the occurrence of crossovers in the two interstitial regions is