Have people ever said to you, "It's in your
genes?" They were probably talking about a physical
characteristic, personality trait, or talent that you share with
other members of your family. We know that genes play an important
role in shaping how we look and act and even whether we get sick.
Now scientists are trying to use that knowledge in exciting new
ways, such as preventing and treating health problems.
What Is a Gene?
To understand how
genes
(pronounced:
jeens
) work, let's review some biology basics. Most living organisms
are made up of cells that contain a substance called
deoxyribonucleic (pronounced: dee-
ahk
-see-rye-bow-noo-
klee
-ik) acid (
DNA
). DNA is wrapped together to form structures called
chromosomes
(pronounced:
krow
-muh-soams).
Most cells in the human body have 23 pairs of chromosomes,
making a total of 46. Individual sperm and egg cells, however, have
just 23 unpaired chromosomes. You received half of your chromosomes
from your mother's egg and the other half from your
father's sperm cell. A male child receives an X chromosome from
his mother and a Y chromosome from his father; females get an X
chromosome from each parent.
So where do genes come in? Genes are sections or segments of DNA
that are carried on the chromosomes and determine specific human
characteristics, such as height or hair color. Because each parent
gives you one chromosome in each pair, you have two of every gene
(except for some of the genes on the X and Y chromosomes in boys
because boys have only one of each). Some characteristics come from
a single gene, whereas others come from gene combinations. Because
every person has from 25,000 to 35,000 different genes, there is an
almost endless number of possible combinations!
Genes and Heredity
Heredity is the passing of genes from one generation to the
next. You inherit your parents' genes. Heredity helps to make
you the person you are today: short or tall, with black hair or
blond, with green eyes or blue.
Can your genes determine whether you'll be a straight-A
student or a great athlete? Heredity plays an important role, but
your environment (including things like the foods you eat and the
people you interact with) also influences your abilities and
interests.
How Do Genes Work?
DNA contains four chemicals (adenine, thymine, cytosine, and
guanine - called A, T, C, and G for short) that are strung in
patterns on extremely thin, coiled strands in the cell. How thin?
Cells are tiny - invisible to the naked eye - and each cell in your
body contains about 6 feet of DNA thread, for a total of about 3
billion miles (if all your DNA threads were stretched out straight)
of DNA inside you! The DNA patterns are the codes for manufacturing
proteins
, chemicals that enable the body to work and
grow
.
Genes hold the instructions for making protein products (like
the enzymes to
digest
food or the pigment that gives your eyes their color). As your
cells duplicate, they pass this genetic information to the new
cells. Genes can be
dominant
or
recessive
. Dominant genes show their effect even if there is only one copy
of that gene in the pair. For a person to have a recessive disease
or characteristic, the person must have the gene on both
chromosomes of the pair.
What Are Genetic Disorders?
Cells can sometimes contain changes or variants in the
information in their genes. This is called
gene mutation
, and it often occurs when cells are aging or have been exposed to
certain chemicals or radiation. Fortunately, cells usually
recognize these mutations and repair them by themselves. Other
times, however, they can cause illnesses, such as some types of
cancer
. And if the gene mutation exists in egg or sperm cells, children
can inherit the mutated gene from their parents.
Researchers have identified more than 4,000 diseases that are
caused by genetic variants. But having a genetic mutation that may
cause disease doesn't always mean that a person will actually
get that disease. Because you inherit a gene from each parent,
having one disease gene usually does not cause any problems because
the normal gene can allow your body to make the normal protein it
needs.
On average, people probably carry from 5 to 10 variant or
disease genes in their cells. Problems arise when the disease gene
is dominant or when the same recessive disease gene is present on
both chromosomes in a pair. Problems can also occur when several
variant genes interact with each other - or with the environment -
to increase susceptibility to diseases.
If a person carries the dominant gene for a disease, he or she
will usually have the disease and each of the person's children
will have a 1 in 2 (50%) chance of inheriting the gene and getting
the disease. Diseases caused by a dominant gene include
achondroplasia (pronounced: ay-kon-druh-
play
-zhuh, a form of dwarfism), Marfan syndrome (a connective tissue
disorder), and Huntington disease (a degenerative disease of the
nervous system).
People who have one recessive gene for a disease are called
carriers, and they don't usually have the disease because they
have a normal gene of that pair that can do the job. When two
carriers have a child together, however, the child has a 1 in 4
(25%) chance of getting the disease gene from both parents, which
results in the child having the disease.
Cystic fibrosis
(a lung disease),
sickle cell anemia
(a blood disease), and Tay-Sachs disease (which causes nervous
system problems) are caused by recessive disease genes from both
parents coming together in a child.
Some recessive genetic variants are carried only on the X
chromosome, which means that usually only guys can develop the
disease because they have only one X chromosome. Girls have two X
chromosomes, so they would need to inherit two copies of the
recessive gene to get the disease. X-linked disorders include the
bleeding disorder
hemophilia
(pronounced: hee-muh-
fih
-lee-uh) and color blindness.
Sometimes when an egg and sperm unite, the new cell gets too
many or too few chromosomes. Most children born with Down syndrome,
which is associated with mental retardation, have an extra
chromosome number 21.
In some cases, people who are concerned that they might carry
certain variant genes can have genetic testing so they can learn
their children's risk of having a disease.
Pregnant women
can also have tests done to see if the fetus they are carrying
might have certain genetic illnesses. Genetic testing usually
involves taking a sample of someone's blood, skin, or amniotic
fluid, and checking it for signs of genetic diseases or
disorders.
Changing Genes
Sometimes scientists alter genes on purpose. For many years,
researchers have altered the genes in microbes and plants to
produce offspring with special characteristics, such as an
increased resistance to disease or pests, or the ability to grow in
difficult environments. We call this
genetic engineering
.
Gene therapy
is a promising new field of medical research. In gene therapy,
researchers try to supply copies of healthy genes to cells with
variant or missing genes so that the "good" genes will
take over. Viruses are often used to carry the healthy genes into
the targeted cells because many viruses can insert their own DNA
into targeted cells.
But there are problems with gene therapy. Scientists haven't
yet identified every gene in the human body or what each one does.
Huge scientific efforts like
The Human Genome
(pronounced:
jee-
nome)
Project
and related projects have recently completed a map of the entire
human genome (all of the genetic material on a living thing's
chromosomes), but it will take many more years to find out what
each gene does and how they interact with one another. For most
diseases, scientists don't know if and how genes play a role.
Plus, there are major difficulties inserting the normal genes into
the proper cells without causing problems for the rest of the
body.
There are also concerns that people might try changing genes for
ethically troubling reasons, such as to make smarter or more
athletic children. No one knows what the long-term effects of that
kind of change would be.
Still, for many people who have genetic diseases, gene therapy
holds the hope that they - or their children - will be able to live
better, healthier lives.
Updated and reviewed by:
Louis E. Bartoshesky, MD, MPH
Date reviewed: January 2007
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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