Chapter 3 - Review of Clinical Human Medical Genetics

Abstract

The human DNA sequence is highly polymorphic; for a typical gene, different people have different specific sequences (or alleles) for that gene. As a result, for the typical protein, different people have different levels of activity in that protein. If the activity of a protein is significantly greater or less than the level of activity that is seen in the typical person, that individual will have a greater or lesser susceptibility to the diseases the protein’s function influences than the typical person does. Whether a high-activity or low-activity gene allele represents a risk-increasing allele or a risk-decreasing allele depends on the specific function the protein performs. While some risk-increasing alleles are relatively common, and some increase the individual’s risk for the associated disease dramatically, most risk-increasing alleles are relatively rare, and most only increase the individual’s risk for the associated disease by a small amount.