Functional And Genetic Analysis Of Individual Differences

The types and numbers of cells that are generated and distributed across the retina represent an explicit, although still incompletely deciphered, summary of a species' relationship with its visual world (Hayes and Brooke, 1990; 0sterberg, 1935; Walls, 1942). These cellular parameters can be modified relatively rapidly in response to changes in selective pressure (Jeffery and Martasian, 1998; Jeffery et al., 2000; Williams et al., 1993). Variation is by no means limited to retina, and there are even more striking quantitative differences in the size and structure of the complex quilt of visual areas that extend from occipital pole into temporal and parietal neocortex of mammals (Chapter 32, Organization of Visual Areas in Macaque and Human Cerebral Carter; Chapter 105, the Evolution of the Visual System in Primates).

In this part, we first consider the functional relevance of variation in numbers of neurons and photoreceptors and review what we are beginning to learn about the genetic basis of individual differences within species. As illustrated by a few specific examples as the end of this part, variation in retinal structure traces back to specific allelic differences that affect developmental processes.

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