In contrast to the common assumption that each new mutant results from a unique, independent mutation event, clusters of identical premeiotic mutant alleles are common. Clusters can produce large numbers of related individuals carrying identical copies of the same new genetic change. By entering the gene pool in multiple copies at one time, clusters can influence fundamental processes of population genetics. Here we report evidence that clusters can increase the arrival and fixation probabilities and can lengthen the average time to extinction of new mutations. We also suggest it may be necessary to reconsider other fundamental elements of population genetic theory.

Using a simple image analysis system, we have established normative data obtained from magnetic resonance imaging (MRI) scans for the relative size and growth of the major structural domains of the brain. Midsagittal scans of the head were analyzed in 95 subjects aged 0 to 20 years. Only scans with no demonstrable structural abnormalities of the brain were utilized. Age-related changes in the relative growth of specific structures were calculated by dividing patients into 5-year age increments. No age-related changes were found in the size of the posterior fossa, supratentorial intracranial space, or cisterna magna relative to the total intracranial vault space. A significant (P less than .01) increase in the size of the corpus callosum relative to that of the supratentorial structures and relative to the total intracranial vault space was noted to occur between the 6- to 10-year-old and 11- to 15-year-old age groups. This may reflect an increase in myelination of the corpus callosum that occurs during this period. A small reduction in the ratio of the posterior fossa to the supratentorial space was found between the 0- to 5-year-old and the 6- to 10-year-old age groups, but the biological significance of this difference is uncertain. It appears from these data that quantitative analysis of MRI scans can be used to draw objective conclusions about the relative sizes and growth of the major brain structures. The development of normative data for these structures and the simplicity of the methodology should have many clinical applications in the assessment of aberrant brain development.

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