Photomicrographs of maize chromosomes carrying the 8–9 translocation, at the time of chromosome pairing (a) and opening out (b), and diagrams (c and d). At the stage in a, heterochromatic regions display differential staining along the lengths and between the chromosomes. a is from McClintock (not previously published), and b is from Creighton (1933). [Reproduced with permission from ref. 9 (Copyright 1934, McGraw–Hill).]

The experimental maize cross and progeny types designed to test the hypothesis that exchange between genes is accompanied by exchange of physical parts of chromosomes. (Upper) a morphologically marked hybrid, knobbed vs. knobless and normal vs. interchanged, and a genetically marked hybrid, C (colored kernels) vs. c (colorless kernels) and Wx (normal starch, blue-staining kernels and pollen) vs. wx (waxy starch, reddish-staining seeds and pollen) is crossed with knobless normal chromosomes bearing c and heterozygous for Wx and wx. One crossover in the critical region is shown in the hybrid, exchanging parts between the genes and between the physical markers. (Lower) the (viable) products resulting from no crossover or a crossover in the critical region. Note that, because the pollen can be classified for Wx vs. wx, progeny plants that are Wx Wx vs. Wx wx vs. wx wx can be distinguished by staining samples of the pollen. Note also that, because plants heterozygous for a translocation have 50% aborted pollen and eggs (Fig. 4), progeny plants can be classified for normal vs. interchange-carrying constitution. (Adapted from ref. 13 to reflect the exact experiment.)

Comparative diagrams of the experiments of Creighton and McClintock with maize and of Stern with Drosophila, mutually supporting the hypothesis of physical and genetic exchange in two key experimental species. [Reproduced with permission from ref. 9 (Copyright 1934, McGraw–Hill).]

Chromosomal constitution and cytological features used in the maize experiment. Centromeres are shown as clear circles. In a, the two normal chromosomes, number 9 (n, barred) bearing a dark-staining knob at the end of its short arm and 8 (N, black) are shown with arrows denoting the point of reciprocal interchange of parts to form a translocation. In b, interchange chromosome 9 (I) has a part of chromosome 8 appended, and chromosome 8 (i) bears a part of chromosome 9. In c, the normal and interchanged chromosomes are shown diagrammatically in the paired cross formation in early meiosis during the process of gamete formation (see Fig. 3). In d, the opening out of the four chromosomes into a ring is diagrammed, from which adjacent or alternate pairs of chromosomes distribute to the division poles, resulting in deficient, inviable gametes or viable ones, respectively. The chromosomes are shown as single rather than double strands in this diagram, for simplicity. For the demonstration of crossing-over, the constitution in c and d required that one chromosome 9 end be knobbed and the other knobless. [Reproduced with permission from ref. 12 (courtesy of Peter McKinley).]

Consequences of distributions of chromosomes from a heterozygous translocation, showing the arising of viable vs. deficient-duplicate, nonviable gametes and the results of self-pollination. Note that there are three classes in such a progeny, occurring in a 1:2:1 ratio: standard chromosome constitution with normal pollen grains, heterozygous translocation with semisterile pollen grains, and homozygous translocation with normal pollen grains, due to a complete but rearranged set of chromosomes. [Reproduced with permission from ref. 9 (Copyright 1934, McGraw–Hill).]

Data presented by Creighton and McClintock, 1931, Table 3. Kernels were classified as C wx (colored waxy kernels, non-crossover—see Fig. 4), cwx (colorless waxy kernels, crossover), CWx (colored non-waxy kernels, crossover), and cWx (colorless non-waxy kernels, non-crossover). Progeny plants may be classified for the knob by cytology at meiosis or in postmeiotic divisions, for the interchange by cytology or by semisterile vs. normal pollen, and for wx constitution by staining samples of pollen with an iodine solution (IKI), as described in Fig. 4. Plants from Class I carried the knob and the interchange, that is, without genetic or physical exchange, while those from Class II were knobless and carried the interchange, that is, with a physical exchange accompanying the genetic exchange. In Class III, those individuals that could be classified showed a physical crossover and accompanying genetic exchange. In Class IV, all of the individuals were knobless (having no exchange between the knob and gene c), and those without physical exchange (Knobless and Normal) were consistent with no genetic exchange, while those with physical exchange (Knobless and Interchanged) were due either to genetic exchanges in the critical region or between wx and the interchange. [Reproduced with permission from ref. 1 (courtesy of Peter McKinley and the Creighton estate).]

Simplified diagram of the cross as presented by Creighton and McClintock, 1931, as a text diagram (p. 495). Note that in this diagram, interchanged chromosome 9 (I) carries the knob and normal chromosome 9 (N) is knobless. Centromeres are not identified, and nonessential segments are excluded. [Reproduced with permission from ref. 1 (courtesy of Peter McKinley and the Creighton estate).]