Crossing over and Anaphase I Distribution of the Chromosomes of a Maize Interchange Trivalent.

PREDICTION of genetic segregation ratios from polyploids and polysomics is usually complicated by the necessity of simultaneous estimation of crossover frequencies and frequencies of the modes of anaphase I distribution. In a special hyperploid maize stock, the frequency of chiasma formation in one arm of a trivalent configuration and the relative frequencies of disjunctive and nondisjunctive distribution of the chromosomes involved in these chiasmata can be estimated genetically with only the one assumption (and this for only part of the data) that random distribution occurs at anaphase 11. This maize stock carries ZeaTripsacum interchange chromosomes such that a trivalent configuration is almost always found at pachytene. Two of the three chromosomes involved in these trivalents regularly disjoin from each other, and preferential pairing effectively limits the type of configuration formed so that crossing over occurs between the remaining chromosome and only one of these two. The genetic constitution has been made such that recessive progeny arise only from such crossing over. The relative frequency of the recessive progeny among 20and 21-chromosome plants is a direct indication of the relative frequencies of disjunctive and nondisjunctive distribution of chromosomes involved in these chiasmata. There are numerous accounts in the literature of deviations from standard recombination frequencies in the multivalents of polyploids, polysomics and translocation configurations. Where a decrease in crossover frequency is found, this is usually attributed to synaptic failure. Thus OLIVER and VAN ATTA ( 1932) noticed that if translocated regions of Drosophila chromosomes paired with their homologues in oogonial cells the crossover frequency was normal or nearly normal, but that otherwise it was reduced. BURNHAM ( 1934) reported similar observations in maize, where crossover frequencies from translocated regions were markedly reduced except in an unusual case where pachytene pairing was complete in the cross configuration found in such structural hybrids. BEADLE (1933), DOBZHANSKY (1930, 1935) and BROWN (1940) found a reduced crossover frequency in translocated regions in Drosophila. RHOADES ( 1931, 1936) reported decreased crossover frequencies in duplicated regions of Drosophila and maize, and DOBZHANSKY reported similar results in Drosophila. UPCOTT (1939) found a lower chiasma frequency per chromosome in tetraploid Primulas than would have been deduced from diploid chiasma frequencies.