Figure 1.

White's constraint hypothesis illustrated with a meiotic configuration of four sex chromosomes (X₁X₂X₃Y, see also Fig. 3) as seen in some male Habronattus. Each chromosome's centromere is shown as a circle, and its chromatids as bars. The Y has two arms, each of which pairs with an X. (A) Chiasmata distal (at tips of chromatids), “loose pairing.” (B) Chiasmata proximal (near the centromere), “tight pairing.” Loose pairing gives enough room for proper segregation (Y to one pole, three Xs to the other); tight pairing is predicted to lead too often to improper disjunction.