Constraints upon the composition of supplementary DNA

Speciation in eukaryotes is often accompanied by massive changes in nuclear DNA amount resulting from the accumulation or deletion of DNA base sequences within chromosomes. Our evidence shows that, among related species, the DNA lost or gained during divergence is of a remarkably consistent and restricted composition in respect of the ratio of repetitive to non-repetitive base sequences. It is argued that the restriction may in part at least be imposed by natural selection, in the sense that the only changes tolerable are confined to DNA fractions of particular composition and organisation.

The effects of three levels of auditory masking on selected vocal characteristics and the frequency of disfluency of adult stutterers

This study was designed to test the hypothesis that the growth function of the masking effect in stuttering behavior is promoted by systematic increases in vocal intensity. Sixteen normal-hearing adult stutterers read aloud in four conditions (quiet and 10-, 50-, and 90-dB sensation levels of continuous white noise). The oral readings were tape recorded. The recorded data were analyzed later to quantify subjects' vocal intensity, stuttering frequency, and reading time in each condition. The results showed that under various levels of masking, there is an inverse relationship between vocal intensity and stuttering frequency. The results of reading time comparisons between conditions were affected by the absolute frequency of stuttering in quiet or the magnitude of the masking effect or both. Finally, changes in vocal intensity were translated into physiological terms and then interpreted in light of how they might promote fluency and reduce stuttering.

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