A Correlation of Cytological and Genetical Crossing-Over in Zea Mays

Abstract

In Zea mays there is a chromosome pair visibly heteromorphic in two regions: (1) an enlarged chromatic knob on the end of the short arm of the chromosome; when one member of the pair possesses a knob and the other does not, the asymmetry is evident; (2) a modification of the long arm of the same chromosome due to an interchange between it and a non-homologous chromosome. Both pairs of contrasting features can be observed in the meiotic prophases. In a plant "heterozygous" for them, cross-overs in the region between them will result in a recombination of these structural features. When a heterozygous plant in which the interchange chromosome possesses the knob and the standard chromosome is knobless is crossed to a standard knobless plant, the combinations knobless-interchange and knobbed-standard observed in a certain proportion of the progeny indicate that chromosomal cross-overs have occurred in the region between the two features responsible for the heteromor-phism. By the simultaneous use of genes known to be in the cytologically marked region of the chromosome, it has been possible to show that the occurrence of genetic crossing-over is correlated with the occurrence of chromosomal crossing-over. Thus, the assumption which has been made that genetic crossing-over is occasioned by the exchange of chromosome segments would seem to be justified.