The fourteen chromosomes of normal races ofPisum sativum form seven pairs at meiosis. All points of association between chromosomes are shown to be chiasmata by the following considerations:
Direct observation of exchange of partners amongst the chromatids.
Alternate loops between points of contact are at right angles.
The frequency and distribution of these points agrees with that of demonstrable chiasmata in other organisms (e.g. Tulipa andStenobothrus).
The method of separation of the pairs at anaphase, the ehromatids associated at the attachment constriction always passing to the same pole, leads to the separation of chromatids in the regions distal to the first point of contact. The regions distal to the second point of contact are either not separated or are asymmetrical, according to the relationship of the exchanges at the two points.
The nature of the associations of four, whether in a ring or a chain, is shown to depend upon the formation of chiasmata between the homologous segments. Its appearance is determined by the number and distribution of chiasmata present and by its orientation.
In about half the cases examined non-disjunction occurs in the ring, two adjacent chromosomes passing to each pole. It is suggested that this gives rise to non-viable gametes through genetic unbalance, brought about by chromosome segment deficiency and reduplication.
Volume 101, 2022
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