A Cytogenetic Study of a Chromosome Fragment in Maize.

ACH of the ten groups of linked genes in maize has been associated E with a specific member of the haploid set of ten chromosomes. The morphology of the ten chromsomes has been studied a t pachytene by MCCLINTOCK (1933) and each member found to differ from the others in such characters as length, position of the spindle fiber insertion region and presence of deeply staining knobs in definite positions, so that they can be recognized by their morphological characteristics. The fifth longest chromosome of the haploid set of maize has a spindle fiber insertion region that is nearly median, dividing the chromosome into two arms. The ratio of the length of the two arms is 1.1 : I .o. In certain strains the longer of the two arms, however, is differentiated a t pachytene by the presence of a deeply staining knob that makes it easy to distinguish between the two nearly equal arms of the chromosome. Plants which possess an extra chromosome V are said to be trisomic for this chromosome. These trisomic plants differ markedly in appearance from their disomic sibs and it is possible to classify a segregating progeny into trisomic and disomic types with considerable accuracy. For this reason plants trisomic for chromosome V have been successfully utilized in an attack on certain problems. There are two secondary trisomes possible for each primary trisome since the extra chromosome in the secondary trisome is composed of two identical arms, that is, one of the arms of the chromosome is represented twice in the extra chromosome. One of the two secondary trisomes for chromosome V has been found (RHOADES 1933b). The extra chromosome consisted of two short arms of chromosome V. This secondary differed strikingly in appearance from both primary trisomes and disomes.