The determination of position in crossing-over - III. The evidence of metaphase chiasmata
The positions of chiasmata at metaphase are dependent on (a) their positions at the time of formation, and (b) subsequent movement. These two factors may now be separated analytically.
A method is given for determining the mean positions of formation of chiasmata relative to the centromere and to one another, by comparison between the size classes in organisms with a large chromosome size range. The original positions as determined in this way seem to be retained at metaphase. There is thus no need to suppose that interstitial chiasmata have moved between their formation and the attainment of metaphase.
Thus terminalization is an all or none process. Either all the chiasmata terminalize or they do not move, except for occasional movement of the most distal chiasma to the end of the bivalent as a result of absorption of the free distal arm.
Some properties of chiasma formation in two-armed chromosomes are discussed and certain observations which would be critical for the hypothesis of centromeric determination of the position of chiasmata are described.
Volume 100, 2021
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