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.
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
illustration: Ida Floreak/Anne
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