1. Of the two theories relating to the mechanism of crossingover, the loop or chiasma-type theory and the synapsis theory, the latter based upon the following four postulates is capable of explaining many relations which have been hardly intelligible in the loop theory.
(a) Crossingover may occur at any point of the portion of the chromosomes which has undergone synapsis.
(b) The process of crossingover is initiated at the distal or free end of the chromosomes and proceeds towards the proximal end.
(c) Interference arises after one break has occurred, and it vanishes suddenly beyond a certain distance from the point of the original break.
(d) Crossingover between sister strands is impossible.
2. From the above postulates, it necessarily follows:
(a) If one considers a chromosome as divided into many small joints and denotes the average number of joints over which interference extends by γ, the coincidence between two regions attains its maximum value at the joint γ+1 apart from the end of the first region, and it decreases gradually with distance (cf. Fig. 4).
(b) In most cases, the maximum of coincidence can exceed 1.00, but as the first and the second regions become longer, it decreases gradually and falls below 1.00 at last.
These two relations have been sufficiently proved by the experiments in
D. virilis and
D. melanogaster.
3. A more plausible explanation than the loop theory can be given for the mechanism of crossingover in the V-shaped chromosomes on the basis of the two assumptions,
viz., (1) one V-shaped chromosome is composed of two rod-shaped chromosomes united at their proximal ends; (2) synapsis is usually incomplete in the middle region of the V-shaped chromosomes.
4. The theoretical results derived from these assumptions lead to the following relations: (a) As the frequency of incomplete synapsis increases the recombination value decreases markedly near the middle portion of the chromosomes, even if the length of the chromosome remains constant. (b) Coincidence between any two successive regions increases abruptly near the middle region of the chromosome, where it exceeds commonly 1.00. (c) Coincidence between one region and other regions reaches its maximum value at a certain distance from the former, then it oscillates off and on as the distance increases.
These relations conform well with what have already been ascertained by experiments carried out by many previous investigators for the second and third chromosome in
D. melanogaster.
5. It has been found that the correction map of the V-shaped chromosomes illustrating the relation between the recombination and the standard values, may be different from that given by MORGAN and his co-workers. The new correction map has been tested and verified by using the data given by BRIDGES, REDFIELD and others.
6. ANDERSON'S data on crossingover in attached X's case afford a valuable material for determining how far the synapsis theory is in harmony with the experimental results. Thus the following conclusions have been established:
(a) Crossingpver occurs in a four-strands stage. (b) Crossingover involves only two of the four strands at any point of crossingover. (c) The first meiotic division in
Drosophila females is reductional and the second is equational for the portion of the spindle-fiber attachment.
View full abstract