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What are genetic disorders?
Both environmental and genetic factors have roles in the development
of any disease. A genetic disorder is a disease caused by abnormalities
in an individual’s genetic material (genome). The four different
types of genetic disorders are(1) single-gene, (2) multifactorial, (3)
chromosomal, and (4) mitochondrial.
(1) Single-gene (also called Mendelian or monogenic)
- This type is caused by changes or mutations that occur in the DNA
sequence of one gene. Genes code for proteins, the molecules that carry
out most of the work, perform most life functions, and even make up
the majority of cellular structures. When a gene is mutated so that
its protein product can no longer carry out its normal function, a disorder
can result. There are more than 6,000 known single-gene disorders, which
occur in about 1 out of every 200 births. Some examples are cystic fibrosis,
sickle cell anemia, Marfan syndrome, Huntington’s disease, and
Single-gene disorders are inherited in recognizable patterns: autosomal
dominant, autosomal recessive, and X-linked. More information on the
different modes of inheritance is available from the following Web sites:
(2) Multifactorial (also called complex or polygenic)
- This type is caused by a combination of environmental factors and
mutations in multiple genes. For example, different genes that influence
breast cancer susceptibility have been found on chromosomes 6, 11, 13,
14, 15, 17, and 22. Its more complicated nature makes it much more difficult
to analyze than single-gene or chromosomal disorders. Some of the most
common chronic disorders are multifactorial. Examples include heart
disease, high blood pressure, Alzheimer’s disease, arthritis,
diabetes, cancer, and obesity. Multifactorial inheritance also is associated
with heritable traits such as fingerprint patterns, height, eye color,
and skin color.
(3) Chromosomal - Chromosomes, distinct structures
made up of DNA and protein, are located in the nucleus of each cell.
Because chromosomes are carriers of genetic material, such abnormalities
in chromosome structure as missing or extra copies or gross breaks and
rejoinings (translocations) can result in disease. Some types of major
chromosomal abnormalities can be detected by microscopic examination.
Down syndrome or trisomy 21 is a common disorder that occurs when a
person has three copies of chromosome 21.
(4) Mitochondrial - This relatively rare type of
genetic disorder is caused by mutations in the nonchromosomal DNA of
mitochondria. Mitochondria are small round or rod-like organelles involved
in cellular respiration and found in the cytoplasm of plant and animal
cells. Each mitochondrion may contain 5 to 10 circular pieces of DNA.
Where can I learn more about different
kinds of genetic disorders?
Disorder Guide -- Find overviews of genetic disorders, Web portals
and directories, bibliographic databases, support groups, genetic health
professionals, genetic testing laboratories, and clinical trials information.
This resource is part of Gene
Gateway, a Web-based guide to learning more about the genes, traits,
and disorders included on the Human Genome Landmarks poster. View poster online.
- National Organization for Rare Disorders -- NORD, a nonprofit organization, serves as an international clearinghouse of information on over 5,000 rare disorders. Refer to their Contact NORD page for telephone, TDD, and other contact information.
- Genetic Alliance -- non-profit organization dedicated to helping individuals and families who have genetic disorders. Tel: 202-966-5557, Fax: 202-966-8553,
Why do we have genes that cause genetic
Many genes are named for the disorders to which they have been linked.
This can be very confusing. For example, the gene associated with hereditary
hemochromatosis is called the “hemochromatosis gene.” This
name implies that the gene exists for the sole purpose of causing disease,
which of course is not the case. The normal function of a gene is to encode
a protein, not cause illness. Disease occurs when genes are unable to
work properly. The hemochromatosis gene actually codes for a membrane
protein that works with other proteins to regulate iron absorption in
cells. Like other single-gene disorders, hemochromatosis occurs when a
gene is mutated in a way that prevents it from encoding a normal, functional
protein product. See hereditary hemochromatosis disorder
profiles for more information about this condition.
If you have been diagnosed
with a genetic disease or have been recommended for genetic testing, your
doctor should refer you to a genetic counselor or medical geneticist.
With specialized backgrounds in medical genetics and counseling, genetic
counselors work with medical personnel (usually M.D. medical geneticists)
to give information, answer questions, and offer support to persons and
families who have genetic disorders, are undergoing gene testing, or may
be at risk for inheriting genetic disorders. They conduct one-on-one counseling
in helping people understand the disease, its implications for their lives
and the lives of family members, and their testing and treatment options.
For more about what genetic counselors do, see the Human Genome Project
Counseling page or What
is a Genetics Consultation? provided by GeneTests.
for Locating Genetic Health Professionals By Area
Society of Genetic Counselors - Use ResourceLink.
Board of Genetic Counseling - Search the Membership
Directory, a combined listing of members of the American Society
of Human Genetics, Genetics Society of America, American College of
Medical Genetics, American Board of Medical Genetics, and American Board
of Genetic Counseling. For a list of all members in a particular city
or state, enter the location without entering any names.
Association of Genetic Counselors
- Medical Geneticists are
M.D.'s, Ph.D.'s, or M.D.-Ph.D.'s with specialized training in the diagnosis
of genetic disorders. To find a medical geneticist in your region, contact
College of Medical Genetics (ACMG) or search the Membership
Directory. For a list of all members in a particular city or state,
enter the location without entering any names.
Where can I learn more about genetic testing --availability, implications,
and background information?
An increasing number of gene tests are becoming available commercially,
although the scientific community continues to debate the best way to
deliver them to the public and to medical communities that are often unaware
of their scientific and social implications. While some of these tests
have greatly improved and even saved lives, scientists remain unsure of
how to interpret many of the test results. Also, patients taking the tests
may face significant risks, including jeopardizing their employment or
insurance status. And because genetic information is shared, these risks
can extend to family members as well. To learn more about gene testing,
see the following resources:
What are some of the ethical, legal, and social implications of being
diagnosed with a genetic disease?
Are patients being properly informed about the risks and limitations
of genetic technology? How does personal genetic information affect an
individual and society's perceptions of that individual? Who owns and
controls genetic information? If you are tested for or diagnosed with
a genetic disorder and this information becomes a part of your medical
record, insurance companies, employers, and other agencies mat be able
to access this information. Without adequate legal measures to protect
individuals from misuse of their medical information, individuals diagnosed
with certain genetic conditions potentially could be denied insurance
coverage, employment, or other benefits.
and Its Impact on Science and Society: The Human Genome Project and
Beyond (2003) - This primer covers basic science, the Human Genome
Project, what we know so far, ELSI, medicine, benefits, future scientific
challenges, and more.
and the New Genetics - From the Human Genome Project Information
Web site. Includes information about new pharmaceuticals and therapies
resulting from genomic research. The site is designed to help physicians,
nurses, genetic counselors, and other medical personnel keep abreast
of genomic medicine.
Science Behind the Human Genome Project - Basic genetics, genome
draft sequence, and post genome science.
Glossary - A glossary of genetic terms from the U.S. Department
of Energy Human Genome Program.
- Genetic Science Learning
Center - An excellent source for online multimedia tutorials and
activities covering such topics as basic genetics, chromosomes, genetic
disorders, cloning, stem cells, and genetic testing. Browser requirements:
Internet Explorer 6, Netscape Navigator 6.x, or Mozilla 1.x.
- Dolan DNA Learning Center (DNALC)
- This Web site from Cold Spring Harbor
Laboratory has numerous educational tools. DNA
Interactive is a multimedia site dedicated to the discovery of the
complementary pairing of DNA bases and the scientists who contributed
to this discovery. Your Genes, Your Health
is an online multimedia encyclopedia of genetic disorders. DNA
From the Beginning is a Web-based educational module that covers
basic concepts of inheritance through up-to-the-minute methods of DNA
analysis. Other resources at this site include an Image Archive on the
American Eugenics Movement; BioServers (bioinformatics tools for students
and teachers); Biology Animation Library; and Restriction Maps and Nucleotide
Sequences of pAMP, pKAN & pBLU Plasmids.
- Blazing a Genetic
Trail - An introduction to research on mutant genes and hereditary
diseases. Published in 1991 from the Howard Hughes Medical Institute.
Know Ourselves - An overview of the Human Genome Project published
in 1996. From the U.S. Department of Energy.
Professional Genetics Societies
- The American
Society of Human Genetics is the primary professional organization
of human geneticists in North America. Current membership includes medical
doctors, research scientists, and genetic counselors. The society publishes
The American Journal of Human Genetics. Address: 9650 Rockville
Pike; Bethesda, MD 20814-3998 (301/571-1825).
- The Genetics
Society of America publishes the scientific journal Genetics,
sponsors several national meetings each year, and cooperates in organizing
an international congress every 5 years under the auspices of the International
Genetic Federation. Address: 9650 Rockville Pike; Bethesda, MD 20814-3998
- The National Society of Genetic Counselors
publishes the Journal of Genetic Counseling and a quarterly newsletter,
Perspectives in Genetic Counseling. The society also sponsors
education conferences and offers members a job connection service. Address:
233 Canterbury Drive; Wallingford, PA 19086 (215/872-7608, Fax: -1192).
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