There are large differences in neuron number both within and between species. This variation in neuron number results from both non-genetic and genetic factors. Non-genetic factors, such as litter size and parity, and genetic cofactors, such as sex, age, and body weight, can influence neuron number. However, the cumulative variance in neuron number that can be explained by the summation of these factors is unknown.
Genetic variation can explain a substantial portion of the variation in neuron number.
However, the identity of the genetic factors and the manner in which they influence neuron number are not known. In this dissertation, I have analyzed the components of environmental and genetic variation contributing to variation in neuron number among inbred strains of mice. I have focused on variation in neuron number on a large scale, by using the surrogate measure of whole brain weight, and variation within a distinct neuron population, retinal ganglion cells. Brain weight ranges from 403 mg to 495 mg among standard inbred strains of mice. I have assessed the relative importance of environmental and genetic factors in the variation of brain weight in 27 standard inbred strains, two sets of recombinant inbred strains, and four F2 intercrosses.