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The majority of studies on heterosis are concerned with observations of quantitative traits whose inheritance is complex. Identification and location of the genetic determiners of these traits would appear a desirable but difficult accomplishment. However, a segment-by-segment analysis of the genome to determine the presence or absence of clusters of such genes should bring us a step closer to the genetic mechanisms underlying the phenomenon of heterosis. A method has been suggested by DOBZHANSKY and RHOADES (1938) in which gates affecting agronomic characters may be located by means of paracentric inversions. In plants heterozygous for such inversions, the crossovers that occur within the inverted segments are only rarely recovered in viable gametes. Thus, the inverted segment is inherited as a unit and all genes contained within the segment are completely linked with very few exceptions. This method is especially advantageous if the agronomic trait being considered is determined by a large number of genes with small effects. In such cases the combined effect of genes located in the inverted segment may permit their detection. In plants without inversions, where crossing over breaks up favorable combinations of minor genes, the existence of such genes is more difficult to demonstrate. By making use of a paracentric inversion with a genetic marker, it is possible to follow a cluster of genes from parent to F, and into the backcross or Fz generations. A comparison is made of plants differing genetically on the average only in the constitution of the specific chromosomal segment, whose length depends on the particular inversion involved. If these plants exhibit significant differences in the characters analyzed, it is assumed that the genes responsible are located in the inverted segment.

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