Genomes are found in cells, the microscopic structures that make up
all organisms. With a few exceptions, each of your body's trillions
of cells contains a copy of your genome: the cells in your muscles,
the cells in your brain, the cells in your blood, and so on.

Imagine all the trillions of genomes in your body, in other people's
bodies, in cedar and apple trees, in walruses, forest mushrooms, and
migrating birds: The whole world is full of genomes.

But if the genome is a commonplace thing, it is also quite powerful.
A genome is information that affects every aspect of our behavior and
physiology. Cooking dinner, digesting your food, talking, singing, sleepingyour
genome has a hand in all these things.

A genome alone can't make a person, because we are also influenced
by where we live, the human culture that surrounds us, and hundreds
of other aspects of our environment. But the fact remains that you can't
make a person without a genome.

Studying the human genome, therefore, is likely to give us insights
into why some people die of heart disease and others die of cancer,
why some people are comfortable schmoozing with a crowd of strangers
and others are paralyzed by shyness, why some people have trouble keeping
weight on while others have trouble keeping it off, and so on.

It's not difficult for scientists to get their hands on the human genome.
They draw a bit of blood from people who volunteer to have their genomes
studied, then use some simple laboratory procedures to break open the
cells in the blood sample and extract the DNA. The body is constantly
making more blood cells, so blood is a renewable genome resource.

Each human being contains a slightly different version of the human
genome, but all human genomes are similar enough that we can learn about
the human genome in general by studying the genomes of one or a few
individual people.

Studying the genome can mean many different things. You can study a very
small part of the genome or the genome as a whole. You can study the sequence
of a gene, the function of a gene, the parts of the genome that regulate
genes, or the DNA outside of genes. You can observe where genes are located
in the genome, or investigate how different genes work together.

The next three chapters discuss several ways how scientists are studying
the genome as a whole: sequencing genomes, mapping genomes, and studying
the variation within genomes. Some of what we learn from this work might
help doctors prevent and treat diseases better. Some of it might simply
celebrate the variety that is the human species, or our unity with other
forms of life. Some of it might open up possibilities we haven't even
thought of yet.