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Chapter 1 A Close-Up View of You
Brain Development
and Intelligence
The way your brain grows, develops, and
functions is affected by genes and factors in your
environment. Your brain development will affect
your intelligence. Intelligence is your capacity
for mental activity. It affects your ability to learn,
understand, reason, and think.
How Your Brain Functions
People are born with over 100 billion nerve
cells in the brain called neurons. See 1-7. The
neurons have many extensions that look like
fingers. These fingers are called dendrites. Nerve
cells send information back and forth through the
dendrites.
The space between the dendrites of two
neurons is called a synapse. The body makes
chemicals in the synapses that allow messages
to be carried from one neuron to another. These
chemicals are called neurotransmitters.
Different neurotransmitters result in different
types of messages. For example, some messages
speed up a body response; others slow it down.
Some chemicals produce a positive feeling, while
others help information flow to different parts
of the brain. The way your body responds is
controlled by the neurotransmitters.
The chemicals that are produced in your
synapses can be affected by both your heredity
and your environment. For example, certain
chemicals are produced when you sleep, and
different chemicals are produced when you are
awake. Other chemicals are produced when you
are in danger or in a stressful situation. These
responses are genetic. Environmental factors can
also affect which chemicals are produced in the
synapses. These factors include the food you eat
and how physically active you are.
How Your Brain Grows and
Develops
Each brain cell connects to thousands of
other brain cells. As you grow and develop, your
brain increases the dendrite connections in the
areas or pathways that are used. Certain stimuli
are needed within the environment for this
development to proceed.
Parts of the brain that are not used are
trimmed back, and those nerve cells die off. This
is where the term use it or lose it applies to brain
development. The critical time for learning to
take place is called a window of opportunity. If the
window is missed, the cells die off and the related
development does not take place.
For example, you are born with the capacity
to learn any language. The pathways of your
brain that are used to learn the language you
speak continue to develop, and many dendrite
connections form. The parts you could use to
learn other languages die off because you do not
use them. After age 10, you are still able to learn
another language. However, you may not speak
it as fluently as you would if you had learned it
earlier.
1-6
A Punnett square illustrates the possible combi-
nations of a pair of inherited genes. The parents in this
example each have one dominant and one recessive gene.
Dominant Versus Recessive Genes
DD = Dominant trait
rD = D dominates
rr = Recessive trait
r D
r rr rD
D rD DD
Think More About It
What are some ways you think genetic
research might help the human race? What
are some possible negative effects?