IT is to be assumed—by analogy with the cell, the nucleus and individual chromosomes—that genes are always derived from parent genes, and are not created de novo from non-genie material. Evolution, as seen in the phenotype, must go hand in hand with an increase of the number of genes. Polyploidy as a method of increase of gene numbers is common in plants, but does not seem to have played an important part in the evolution of animals. The work of Bridges1 on salivary gland chromosomes in Drosophila shows that small duplications, often adjacent, are common in Drosophila. They appear as repeat sequences of very similar or identical bands ; when extensive enough, these repeats have a tendency to show somatic pairing within the individual chromosome.
Bridges, C. B., J. Heredity, 26 (1935).
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GRÜNEBERG, H. Gene Doublets as Evidence for Adjacent Small Duplications in Drosophila. Nature 140, 932 (1937). https://doi.org/10.1038/140932b0
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