. The Mechanism of Mendelian heredity. Mendel's law; Heredity; Chromosomes. 166 THE CHEOMOSOMES method by which the strands are found joined at the time when they draw apart, that cross union of the threads must have previously taken place. If crossing over be supposed to take place between two single threads (Fig. 24) all four gametes that ultimately result from such a cell will be crossover gametes. On the other hand, if crossing over takes. /i B c D Fig. 47.—Four stages in crossing over, according to the "typical" chiasma type of Janssens. The white rod and the black rod are each
. The Mechanism of Mendelian heredity. Mendel's law; Heredity; Chromosomes. 166 THE CHEOMOSOMES method by which the strands are found joined at the time when they draw apart, that cross union of the threads must have previously taken place. If crossing over be supposed to take place between two single threads (Fig. 24) all four gametes that ultimately result from such a cell will be crossover gametes. On the other hand, if crossing over takes. /i B c D Fig. 47.—Four stages in crossing over, according to the "typical" chiasma type of Janssens. The white rod and the black rod are each split lengthwise; crossing over takes place only between two of the four strands. place by means of the chiasmatype (Fig. 47) only two of the resulting four cells will be crossover gametes, the other two being non-crossover gametes.^ Looked at from the point of view of the total output, there would be no way in which to tell whether one or the other of the above processes has taken place; although the formation of a given number of crossover gametes involves only half as many participating cells in the case of the single thread type as in the case of the double thread type. 1 If, after the thick threads have split, crossing over involving holh, strands of each chromosome should take place, instead of only one strand as in the chiasmatype, sensu strictu, all four gametes that result would be crossover Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Morgan, Thomas Hunt, 1866-1945. New York, H. Holt
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