Centromere

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.”

The aberrant bivalents with single terminal chiasmata are specially liable to this failure of orientation, presumably because their centromeres lie exceptionally far apart, as is the case in similar circumstances in a Lilium hybrid with reduced chiasma-frequency.

Every chromosome has a long arm and a short arm separated by a pinch point known as a centromere.