Harvard Public Health Review Winter 2007
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A New Twist on Inherited Cancer Risk
Genes and disease risk: A closer look

The human genome is made up of 23 pairs of chromosomes. Each chromosome consists of one long DNA molecule, shaped like a twisted ladder.

The ladder’s rungs are formed by pairs of nucleotides—the chemical bases adenine (A), guanine (G), cytosine (C), and thymine (T). These combine as AT, TA, GC, or CG.

Genes are stretches of DNA containing instructions for making proteins, which orchestrate a cell’s activities. Human cells contain an estimated 22,000 genes. Changes in one or more genes, either inherited or accumulated over time, can alter the course of life for good or ill.

DNA variant illustration

Gene variants are subtle differences in DNA that are carried within populations from one generation to the next. Although most of these variants permit genes to operate relatively normally, their presence may be a red flag for risk of disease caused by a slight change in the gene’s function. (By contrast, mutations are relatively rare genetic abnormalities; they may raise or lower disease risk by grossly interfering with a gene’s function.)

The most common type of gene variant is called a SNP (“snip,” short for single nucleotide polymorphism). Found all along chromosomes, SNPs are single-base variations, points where one person has an “A” while most people have a “T,” for example. Scientists have found about 500,000 of them to be especially useful as signposts that either point to a gene or signal that one is nearby.

illustration: Ida Floreak/Anne Hubbard

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