. The elements of genetics. Genetics. Ml MELANOGASTER SIMULANS se St p ap AH 1 ^ t ca J06 J34 se St HA ap P ca Fig. 31.—The right arm of chromosome III of Drosophila sitmilans has a. segment which is inverted relative to the corresponding chromosome arm in D. melanogaster. The effect of this inversion may be seen cytologically in the pairing of the chromo- some arm in saUvary gland nuclei of the species hybrid (above) or genetically in the order of homologous genes when mapped by the crossing-over percentages occurring within each of the species (below). It will be observed also that crossing-
. The elements of genetics. Genetics. Ml MELANOGASTER SIMULANS se St p ap AH 1 ^ t ca J06 J34 se St HA ap P ca Fig. 31.—The right arm of chromosome III of Drosophila sitmilans has a. segment which is inverted relative to the corresponding chromosome arm in D. melanogaster. The effect of this inversion may be seen cytologically in the pairing of the chromo- some arm in saUvary gland nuclei of the species hybrid (above) or genetically in the order of homologous genes when mapped by the crossing-over percentages occurring within each of the species (below). It will be observed also that crossing- over is freer in chromosome III of simulatis than mdanogaster, the interval se-ca (which includes the whole of the inversion) being 127 units as compared with 80 (after Patau, 1935). distinguish different species (Fig. 31). A true inversion, one which does not include the centromere, leads to a pairing of the two relatively changed segments the wrong way round with respect to the centromere. The inverted segments pair in a loop (Fig. 30). Any single crossing-over within this loop gives two new chromatids, one with two centromeres attached to it, a dicentric, the other with none, an acentric. The consequences of this structure are seen at anaphase: the acentric chromatid is left helpless on the equator and 131. 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 Darlington, C. D. (Cyril Dean), 1903-; Mather, Kenneth. New York : Macmillan
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