Anumber of single genes mutants in mice have been discovered that all influence the development of the cerebellum. the weaver mutant shows granule cells in the external granule cell layer instead of in internal granule cell layer. the external layer is where cell division takes place and the internal layer is where mature granule cells are positioned to form neural circuits. which developmental process is compromised by the weaver mutation?
a. cell division
b. cell migration
c. trophic interactions: circuit validation
d. regional patterning
e. none of the above

B

Explanation:

The weaver mutation results in the degeneration of neuronal populations as diverse as granule cells of the cerebellum and dopaminergic neurons in the substantia nigra, while other cells of the CNS are spared. Surprisingly, it is a mutation resulting in the functional alteration of a potassium channel expressed throughout the CNS that results in this rather specific neuronal degeneration.

Analysis of the less severely affected heterozygotes by electron microscopy and autoradiography indicates that granule cell death occurs several days after cell genesis and is secondary to failure of their somas to migrate across the molecular layer to the granular layer. This migration defect in turn appears secondary to a hitherto unrecognized disorder of Bergmann glial cells, the cells that normally guide the young neurons in their migration

source:

Identification of the weaver Mouse Mutation: The End of the Beginning

Ellen J Hess: https://www.cell.com/fulltext/S0896-6273(00)80133-6