Branched chromosomes as symmetrical duplications
1. The bulb of theX-chromosome ofD. melanogaster and branched chromosomes in general are interpreted to be symmetrical duplications formed by a continuous unbranched chromonema.
2. An explanation of the formation of symmetrical duplications is advanced, and evidence for a single event producing more than one break is presented.
3. The bulb antedates the evolutionary split ofD. melanogaster andD. simulans, since it is present in both species. The homology of the two arms of the duplication and therefore of the duplicated genes themselves, could be preserved by the possibility of crossing-over which exists in this type of rearrangement.
4. Some of the observed structural features are referred to certain internal forces in the chromosomes, such as the attraction between homologous chromomeres and the tension within the chromonemas.
5. Sexual dimorphism in regard to theX-chromosome in general and to the bulb in particular has been described. This is interpreted as an expression of a difference in the force of synaptic attraction in the cells of male and female.
Volume 100, 2021
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