. The physical basis of heredity . Heredity. CEOSSING OVEE AND CHEOMOSOMES 111 the linear order of the material of the chromosomes cannot be overestimated. As a further example Wenrich gives identical stages of the same chromosomes (Fig. 49), each of the figures is from a different individual. The identity in size and in location of the principal beads in the series is obvious. Eobertson has also brought forward a case of an unequal pair of chromosomes and interpreted the facts as opposed to the crossing-over hypothesis. He found two cases in a grasshopper of the genus Tettigidea in which ther
. The physical basis of heredity . Heredity. CEOSSING OVEE AND CHEOMOSOMES 111 the linear order of the material of the chromosomes cannot be overestimated. As a further example Wenrich gives identical stages of the same chromosomes (Fig. 49), each of the figures is from a different individual. The identity in size and in location of the principal beads in the series is obvious. Eobertson has also brought forward a case of an unequal pair of chromosomes and interpreted the facts as opposed to the crossing-over hypothesis. He found two cases in a grasshopper of the genus Tettigidea in which there was a very unequal pair of chromosomes. The shorter piece conjugated consistently with only one part of the longer chromosome, as shown in the next figure. a FiQ. 50—Conjugation of an unequal pair of chromosomes and their subsequent separation. (After Robertson.) (Fig. 50, a,b). At the first maturation division the two chromosomes separated, as shown in (c, d, e). It would be diflScult to find a more excellent illustration of the per- sistence of the individuality of the chromosomes after con- jugation, and the case falls equally in line with the view that conjugation takes place only between those parts of the chromosome that are alike, , composed of the same series of genes. How, then, could this case, so admirably suited to support the chromosome theory be turned against the chiasma theory ? Only, I think, through a misconception of the essence of the theory, Eobertson says: "In both types of unequal tetrads we have very strong evidence that the homologous chromosomes, on entering the side-to-side pairing process of synapsis, remain as distinct individ- uals, retain their identity throughout the period, and come. 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.
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