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But in important ways, we also are each unique. We are each a one-of-a-kind package
of looks, language, personality, interests, and cultural background. What causes our striking
diversity? How much is our individuality shaped by our differing genes, and how much
by our environment — by every external influence, from maternal nutrition while in the
womb to social support while nearing the tomb? How does our heredity interact with our
experiences to create both our universal human nature and our individual and social diver-
sity? Such questions intrigue behavior geneticists.
Genes: Our Codes for Life

Barely more than a century ago, few would have guessed that every cell nucleus in your body
contains the genetic master code for your entire body. It’s as if every room in Dubai’s Burj
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Khalifa (the world’s tallest structure) contained a book detailing the architect’s plans for the
entire structure. The plans for your own book of life run to 46 chapters — 23 donated by your
mother’s egg and 23 by your father’s sperm. Each of these 46 chapters, called a chromosome,
Copyright © Bedford, Freeman & Worth Publishers.
is composed of a coiled chain of the molecule DNA (deoxyribonucleic acid). Genes, small
segments of the giant DNA molecules, form the words of those chapters (Figure 1.1-1).
Altogether, you have some 20,000 genes, which are either active (expressed) or inactive. Envi-
ronmental events “turn on” genes, rather like hot water enabling a tea bag to express its
flavor. When turned on, genes provide the code for creating protein molecules, our body’s
building blocks.

DNA
Figure 1.1-1
The life code
Chromosome The nucleus of every human cell
contains chromosomes, each of
which is made up of two strands
of DNA connected in a double
helix. Genes are DNA segments
that, when expressed (turned
Gene
Cell on), direct the development of
proteins that influence a person’s
individual development.

environment every nongenetic
influence, from prenatal nutrition
Nucleus to our experiences of the people
and things around us.
heredity the genetic transfer of
characteristics from parents to
offspring.
genes the biochemical units of
heredity.
Genetically speaking, every other human is nearly your identical twin. Researchers genome the complete
exploring the human genome have discovered the common sequence within human instructions for making an
DNA. This shared genetic profile is what makes us humans, rather than tulips, bananas, or organism.
chimpanzees.

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