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Genetic Diseases

Genetic Diseases

A genetic disease is due to a faulty gene or group of genes. While not all gene defects cause disease, many do. New genetic diseases are discovered every month; as of 2001, there are estimated to be approximately 1,100 genetic diseases.

How Gene Defects Cause Disease

A gene is a recipe for making a protein . Proteins control cell functions, and defects in the instructions for making a protein can prevent the cell from functioning properly. Genes are made of deoxyribonucleic acid (DNA), a chemical composed of units called nucleotides , and are carried on chromosomes within the cell nucleus . Most genes are present in pairs (corresponding to the two sets of chromosomes inherited from one's parents). As well as coding for proteins, genes are the hereditary material. Therefore, genetic diseases can be inherited.

Genetic defects cause diseases in a variety of ways. The simplest way is through a "loss-of-function" mutation. In this type of defect, a change in the DNA nucleotides prevents the gene from making protein, or prevents the protein from functioning once it is made. Genetic diseases due to loss-of-function mutations are very common, and include cystic fibrosis (which affects the lungs and pancreas), Duchenne muscular dystrophy, and the hemophilias, a group of blood-clotting disorders.

A second mechanism for causing disease is called a "toxic-gain-of-function" mutation. In this type of defect, the gene takes on a new function that is harmful to the organismthe protein produced may interfere with cell functions, or may no longer be controllable by its normal regulatory partners, for instance. Many degenerative diseases of the brain are due to this type of mutation, including Huntington disease.

More complex mechanisms are possible. Most traits are multifactorial, meaning they are determined by many different genes. In the human population, there are several variants (alleles) of most genes, each form of which is functional and does not cause disease by itself. However, some alleles may predispose a person to a certain disease, especially in combination with other alleles or environmental factors that influence the same trait. Such susceptibility alleles have been found in breast cancer and colon cancer, for instance. Carriers of these alleles have an increased likelihood of developing that disease, a risk that can be increased or decreased by such factors as diet, exposure to environmental toxins, or presence of particular alleles for other genes. As more is learned about the human genome , a large number of susceptibility genes are likely to be discovered for a wide variety of conditions.

Disease can also be caused by chromosome abnormalities rather than gene defects. Down syndrome is due to having three copies of chromosome 21, instead of the normal two copies. It is likely the extra protein from the extra gene copies lead directly to the disease symptoms, but this is not yet clear.

Condition Chromosome Location and Inheritance Pattern Protein Affected Symptoms and Comments
Gaucher Disease 1, recessive glucocerebrosidase, a lipid metabolism enzyme Common among European Jews. Lipid metabolism enzyme accumulation in liver, spleen, and bone marrow. Treat with enzyme replacement
Achondroplasia 4, dominant fibroblast growth factor receptor 3 Causes dwarfism. Most cases are new mutations, not inherited
Huntington's Disease 4, dominant huntingtin, function unknown Expansion of a three-nucleotide portion of the gene causes late-onset neurodegeneration and death
Juvenile Onset Diabetes 6,11,7, others IDDM1, IDDM2, GCK, other genes Multiple susceptibility alleles are known for this form of diabetes, a disorder of blood sugar regulation. Treated with dietary control and insulin injection
Hemochromatosis 6, recessive HFE protein, involved in iron absorption from the gut Defect leads to excess iron accumulation, liver damage. Menstruation reduces iron in women. Bloodletting used as a treatment
Cystic Fibrosis 7, recessive cystic fibrosis transmembrane regulator, an ion channel Sticky secretions in the lungs impair breathing, and in the pancreas impair digestion. Enzyme supplements help digestive problems
Friedreich's Ataxia 9, recessive frataxin, mitochondrial protein of unknown function Loss of function of this protein in mitochondria causes progressive loss of coordination and heart disease
Best Disease 11, dominant VMD2 gene, protein function unknown Gradual loss of visual acuity
Sickle Cell Disease 11, recessive hemoglobin beta subunit, oxygen transport protein in blood cells Change in hemoglobin shape alters cell shape, decreases oxygen-carrying ability, leads to joint pain, anemia, and infections. Carriers are resistant to malaria. About 8% of US black population are carriers
Phenylketonuria 12, recessive phenylalanine hydroxylase, an amino acid metabolism enzyme Inability to break down the amino acid phenylalanine causes mental retardation. Dietary avoidance can minimize effects. Postnatal screening is widely done
Marfan Syndrome 15, dominant fibrillin, a structural protein of connective tissue Scoliosis, nearsightedness, heart defects, and other symptoms
Tay-Sachs Disease 15, recessive beta-hexosaminidase A, a lipid metabolism enzyme Accumulation of the lipid GM2 ganglioside in neurons leads to death in childhood
Breast Cancer 17, 13 BRCA1, BRCA2 genes Susceptibility alleles for breast cancer are thought to involve reduced ability to repair damaged DNA
Myotonic Dystrophy 19, dominant dystrophia myotonica protein kinase, a regulatory protein in muscle Muscle weakness, wasting, impaired intelligence, cataracts
Familial Hypercholesterolemia 19, imcomplete dominance low-density lipoprotein (LDL) receptor Accumulation of cholesterol-carrying LDL in the bloodstream leads to heart disease and heart attack
Severe Combined Immune Deficiency ("Bubble Boy" Disease) 20, recessive adenosine deaminase, nucleotide metabolism enzyme Immature white blood cells die from accumulation of metabolic products, leading to complete loss of the immune response. Gene therapy has been a limited success
Adrenoleukodystrophy X lignoceroyl-CoA ligase, in peroxisomes Defect causes build-up of long-chain fatty acids. Degeneration of the adrenal gland, loss of myelin insulation in nerves. Featured in the film Lorenzo's Oil
Duchenne Muscular Dystrophy X dystrophin, muscle structural protein Lack of dystrophin leads to muscle breakdown, weakness, and impaired breathing
Hemophilia A X Factor VIII, part of the blood clotting cascade Uncontrolled bleeding, can be treated with injections or replacement protein
Rett Syndrome X methyl CpG-binding protein 2, regulates DNA transcription Most boys die before birth. Girls developmental retardation, mutism, and movement disorder
Leber's Hereditary Optic Neuropathy mitochondria, maternal inheritance respiratory complex proteins Degeneration of the central portion of the optic nerve, loss of central vision
Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke (MELAS) mitochondria, maternal inheritance transfer RNA Recurring, stroke-like episodes in which sudden headaches are followed by vomiting and seizures; muscle weakness

Inheritance Patterns in Genetic Disease

Genetic diseases are heritable, meaning they may be passed from parent to child. A disease gene is called recessive if both copies of the gene must be defective to cause the disease. Loss-of-function mutations are often recessive. If the second copy of the gene is healthy, it may be able to serve adequately even if the first copy suffers a loss-of-function mutation. In this case, the carrier of the disease gene will not have the disease.

All humans are thought to carry a number of such defective genes. Close relatives are likely to carry similar genes and gene defects, and are therefore more likely to bear children with recessive genetic diseases if they mate. Because of this, a prohibition against marriage of close relatives is found in virtually every culture in the world.

A disease gene is called dominant if inheriting one copy of it causes the disease. Toxic gain-of-function mutations often create dominant genes, as in the case of Huntington disease.

If having one defective gene causes a different condition than having two, the gene is called incompletely dominant. In familial hypercholesterolemia, having two disease genes leads to very high blood cholesterol levels and death in childhood or early adulthood. Having one disease gene and one normal gene leads to less-elevated cholesterol and a longer but still reduced life span.

Most genes are carried on autosomes, the twenty-two pairs of chromosomes that do not determine sex. Males and females are equally likely to inherit disease genes on autosomes and develop the related diseases, called autosomal disorders. Unlike autosomes, the pair of chromosomes that determine sex (called X and Y) have almost no genes in common. While the Y carries very few genes, the very large X chromosome contains many genes for proteins unrelated to sex determination. Males have one X and one Y, and are more likely than females to develop diseases due to recessive X-linked genes, since they do not have a backup copy of the normal gene. Such disorders are termed X-linked disorders. Females have two X chromosomes, and so usually do not develop recessive X-linked disorders. Duchenne muscular dystrophy, for instance, is an X-linked condition due to a defective muscle protein. It affects boys almost exclusively. Females are carriers for the condition, meaning they have the gene but seldom develop the disease.

The cell energy organelles called mitochondria also contain a small number of genes. Mitochondria are inherited only from the mother, and so mitochondrial gene defects show maternal inheritance. Leber's hereditary optic neuropathy is a maternally inherited mitochondrial disorder causing partial blindness.

In some diseases, not every person who inherits the gene will develop the disease. Such genes are said to show incomplete penetrance. For instance, fragile X syndrome does not affect about one-fifth of boys who inherit it. This syndrome is due to a large increase in the number of CCG nucleotides at the tip of the X chromosome and leads to characteristic facial features, mental retardation, and behavioral problems.

Unique Features of Genetic Diseases

If a parent is known to carry a disease gene, it is possible to predict the likelihood that an offspring will contract the disease, based on simple laws of probability. In Duchenne muscular dystrophy, for instance, if the mother carries the defective gene, there is a 50 percent chance that each male child will develop the disease, since she will give the child one of her two X chromosomes. It is also possible with many disorders to test the fetus to determine if the gene was in fact inherited. Such information can be used for purposes of family planning.

Different populations may have different frequencies of disease alleles because of long periods of relative genetic isolation. For instance, Jews of European ancestry are much more likely to carry the gene for Tay-Sachs disease, a fatal autosomal recessive disorder of lipid metabolism . Healthy adults in such populations may choose to be tested to see if they carry one Tay-Sachs allele. A person with one disease allele might use this information to avoid choosing a mate who also has one disease allele.

Treatment of genetic diseases is possible in some but not all cases. Missing proteins can be supplied relatively easily to the blood, as for hemophilia, but not to most other organs. The effects of phenylketonuria, which is due to a defect in an enzyme that breaks down phenylalanine, can be partially avoided by reducing the amount of the amino acid phenylalanine in the diet. (This is the reason some diet soft drinks carry a notice that phenylalanine is used in the artificial sweetener.) Most genetic diseases can't be treated, though, except by supplying the missing gene to the tissues in which it acts. This treatment, called gene therapy, is still experimental, but may become an important type of therapy for genetic diseases in the coming decades.

see also Gene Therapy; Genetic Analysis; Genetic Counselor; Mutation; Patterns of Inheritance; Pedigrees and Modes of Inheritance; Sex Chromosomes

Richard Robinson

Bibliography

Bellenir, Karen. Genetic Disorders Sourcebook. Detroit, MI: Omnigraphics, 1996.

Genes and DiseaseInformation and Chromosome Maps from National Institutes of Health. <http://www.ncbi.nlm.nih.gov/disease/>.

Lewis, Ricki. Human Genetics: Concepts and Applications, 4th ed. New York: McGraw-Hill, 2001.

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Genetic Diseases

Genetic Diseases

What Is Heredity?

What Causes Genetic Diseases?

How Are Diseases Inherited?

What Are the Common Inheritance Patterns of Genetic Diseases?

The Past and Future of Genetic Diseases

Resources

Genetic diseases are disorders that are inherited by a person from his or her parents or are refaed to some type of spontaneous genetic change.

KEYWORDS

for searching the Internet and other reference sources

Birth defects

Gene therapy

Genetic counseling

Genetics

Hereditary diseases

Human genome

Prenatal diagnosis

What Is Heredity?

Every person develops under the influence of a mix of genes inherited from his or her mother and father. These genes, or small parts of chromosomes, determine the architecture and activity of the entire body. They determine visible characteristics, such as eye color, skin color, and height, as well as traits that cannot be seen, such as the likelihood of certain diseases, the chemicals made by the body, and the functioning of body systems.

Normally, each cell in the body contains two copies of each gene: one that originally came from the egg of the mother and one from the sperm of the father. In many instances, these two copies are slightly different from each other. The result is a child who has some characteristics from the mother and some from the father, but who is never identical to either parent.

Because there are two copies, a gene that works normally usually can make up for one that has a defect. For example, a gene with a defect that causes a particular disease may be passed through generations of a family without causing illness. That is because the normal gene in the pair may work well enough to mask the defect. However, if a child inherits two genes with the defect, the child will develop the illness. This explains how a child with the disease can be born to parents without it.

What Causes Genetic Diseases?

Genetic disorders can be inherited, in which case people are born with them, even if they are not noticeable at first. Some disorders, however, are not inherited but develop spontaneously when disease-causing mutations* occur during cell division*. These also are genetic disorders, because they involve changes in the genes.

* mutations
(mu-TAY-shuns) are changes in a chromosome or a gene.
* cell division
is the process by which a cell divides to form two daughter cells, each of which contains the same genetic material as the original cell.

Some inherited genetic disorders, such as cystic fibrosis* and phenylketonuria* (PKU), are caused simply by the inheritance of genes that do not work properly. In other disorders, however, genetic and environmental factors seem to work together to cause changes in otherwise normal genes. For example, some forms of radiation or chemicals can cause cancer in people who are prone to be affected because of their genetic makeup.

* cystic fibrosis
(SIS-tik fi-BRO-sis) is a genetic disorder of the bodys mucus-producing glands. It mainly affects the respiratory and digestive systems of children and young adults.
* phenylketonuria
(fen-ul-ke-ton-U-ree-a), or PKU for short, is a genetic disorder of body chemistry that if left untreated, causes mental retardation.

How Are Diseases Inherited?

Wie beginning of modern genetics Gregor Mendel (1822-1884) is considered the father of modern genetics*. Mendel was an Austrian monk. While growing peas in the monastery garden, Mendel noted that certain traits appeared in offspring in predictable patterns, and he began to understand the basic rules of inheritance. These rules are called Mendelian (men-DEL-ee-an) law.

* genetics
is the branch of science that deals with heredity and the ways in which genes control the development and maintenance of organisms.

Under Mendelian law, a dominant (DOM-i-nant) trait is one that appears even when the second copy of the gene for that trait is different. For example, for the seeds of Mendels peas, smooth is dominant over wrinkled. Thus, if a pea plant contains one gene for smooth and one for wrinkled, the seed will be smooth. Wrinkled is a recessive (re-SES-iv) trait, which is one that only appears when two copies of it are present.

A Genetic Glossary

  • Cells: The units that comprise living beings. The human body is made of about 60 trillion cells.
  • Nucleus: A membrane-bound structure inside cells that contains DNA.
  • Chromosomes: DNA is packaged into units called chromosomes. Humans have 23 pairs of chromosomes, for a total of 46.
  • DNA (deoxyribonucleic acid): A double-stranded molecule, made of chemical bases called nucleotides, that contains the genetic code necessary to build a living being.
  • Genes: Segments of DNA located on the chromosomes. Genes are the units of heredity. They help determine a persons characteristics, from eye color to how various chemicals work in the body.
  • Genome: An animals entire collection of genes. The human genome contains 50,000 to 100,000 genes.

Dominant and recessive genes

Normally, each person has two copies of every gene, one from the mother and one from the father. A physical feature or a disorder carried by genes can be either a dominant (G) or a recessive (g) trait. If the affected gene is dominant, a person with one or two copies of the gene will have the disorder. Therefore, a person with the patterns (GG) or (Gg) will be affected, but (gg) will not be affected by the disorder. Two copies of a dominant gene produce a much more serious form of the disorder.

If the affected gene is recessive, only a person with two copies of the gene will have the disorder. Therefore, a person with the pattern (gg) will be affected, but (GG) and (Gg) will not be affected by the disorder.

Autosomal and sex-linked traits

Of the 23 pairs of chromosomes in human cells, 22 are autosomes (AW-to-somes), or non-sex chromosomes. The other pair contains the two sex chromosomes, which determine a persons gender. Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The reproductive cells, or eggs and sperm, each have only one set of 23 chromosomes. While an egg always carries an X chromosome, a sperm cell can carry either an X or a Y, so it is the sperm that determines gender.

Inherited genetic disorders that are carried on the sex chromosomes are referred to as sex-linked. Disorders carried on the other chromosomes are referred to as autosomal (aw-to-SOME-al). In general, autosomal disorders are likely to affect males and females equally, but sex-linked disorders usually affect males more often than females. This gender difference has to do with the fact that males have only one X chromosome. The X chromosome

carries genes for which there is no second copy on the Y. Therefore, a male has only one copy of these genes. If his copy is damaged or defective, he has no normal copy to override or mask the defective one. Depending on the problem with the gene, the result can be an X-linked disorder.

What Are the Common Inheritance Patterns of Genetic Diseases?

Single-gene autosomal diseases

Most genetic disorders are caused by defective genes on the autosomes. If an autosomal genetic disorder is caused by a problem with a single gene, then the following rules of inheritance usually apply. There are exceptions to these rules, but they are useful guidelines for understanding inheritance. In an autosomal dominant disorder:

  • It takes only one copy of the gene to cause the disorder. So if a child inherits the disease, at least one of the parents has the disease as well.
  • It is possible for the gene to change by itself in the affected person. This change is called a mutation.
  • Unaffected children of a parent with the disorder have unaffected children and grandchildren.

In an autosomal recessive disorder:

  • If two people without the disorder have a child with the disorder, both parents carry one copy of the abnormal gene.
  • If a person with the disorder and a carrier* have a child, there is a
  • fifty-fifty chance that the child will have the disorder. Any child
  • without the disorder will be a carrier.
  • If a person with the disorder and a noncarrier have children, all of
  • the children will be carriers but will not have the disorder.
  • If two people with the disorder have children, all of the children will
  • have the disorder.
* carrier
is a person who has one copy of the defective gene for a recessive disorder. Carriers are not affected by the disorder, but they can pass on the defective gene to their children.

Single-gene sex-linked diseases

More than 150 disease traits are carried on the X chromosome. X-linked dominant disorders are rare. In an X-linked recessive disorder:

  • Nearly all people with sex-linked disorders are male. The disorder is transmitted through the female, because a sons X chromosome always comes from his mother. She is unaffected, however, because she has a second X chromosome which usually contains a normal gene for the trait.
  • A male with the disorder never transmits it to his sons, because a father passes his X chromosome only to his daughters.
  • A son born to a female carrier has a fifty-fifty chance of having the disorder.
  • All daughters of an affected male will be carriers.

Multiple-gene diseases

Many disorders are exceptions to the Men-delian laws of inheritance. Genetic disorders caused by a combination of many genes are called multifactorial (mul-tee-fak-TOR-e-al) disorders. In addition, some disorders show reduced penetrance (PEN-e-trance), which means that a gene is not wholly dominant or recessive. For example, a person who has one recessive gene for a disorder might show milder symptoms of the disorder, but someone with two copies will have the full-blown disorder.

Chromosome disorders

Other genetic disorders are caused by extra or missing chromosomes. In Down syndrome*, a person has three copies of chromosome 21, rather than the usual two copies. In a disease called cri du chat*, a piece of chromosome 5 is missing. In Turner syndrome*, which affects only girls, all or part of an X chromosome is missing. In most cases, chromosome disorders are not inherited. Instead, the problems occur for unknown reasons when the egg and sperm meet to form the embryo.

* Down syndrome
is a genetic disorder that can cause mental retardation, shortness, and distinctive facial characteristics, as well as many other features.
* cri du chat
(kree-doo-SHA), French for cats cry, is a genetic disorder that can cause mental retardation, a small head, and a cat-like whine.
* Turner syndrome
is a genetic disorder that can cause several physical abnormalities, including shortness, and lack of sexual development.

Spontaneous (new) genetic mutations

Particularly in the case of dominantly-inherited disorders, a child may be born with a condition despite the fact that neither parent has the disorder as would be expected. When this happens, it is usually because a spontaneous (or new) mutation in a gene or genes has occurred. The mutation may occur in a parent s egg or sperm cell, or it may occur after the egg has been fertilized and begins to develop into an embryo. This is frequently the case in achondroplasia (a-kon-dro-PLAY-zha), a form of dwarfism in which 90 percent of children born with the condition have unaffected parents. When this child grows up, the child will pass the gene on to his or her children according to the autosomal dominant inheritance pattern described above.

The Past and Future of Genetic Diseases

Mendel figured out the basic concepts of inheritance in the 1800s, before people knew that genes are the units of inheritance. It was not until 1953 that the structure of DNA was described. From the 1980s to the present, scientists understanding of genes and how they work has grown at an incredibly rapid pace. Many disease-causing genes now have been identified, opening the door to research on ways to fix genetic defects. This field of science is referred to as gene therapy.

Gene therapy

Genetic disorders can be treated in a number of ways. In some disorders, special diets are used to prevent the buildup in the body of compounds that are toxic to patients. In other disorders, the treatment involves blocking or rerouting chemical pathways. A third kind of treatment is new and controversial. It involves actually replacing defective genetic material with normal genetic material inside the cells. Researchers currently are looking for ways to do this. A variety of methods are being considered, including the use of microscopic bullets coated with genetic material and viruses to deliver normal genes to cells.

Prenatal testing

A fetus* can be tested for many genetic disorders before it is born. Tests for prenatal (before birth) diagnosis are done on samples taken from the tissue or fluid surrounding a fetus. The fetuss chromosomes then can be studied using a karyotype (KAR-e-o-type), which is a visual display of the chromosomes from cells viewed under a microscope. Newer techniques enable scientists and doctors to look directly at the DNA that makes up the genes contained in the chromosomes. Common prenatal tests include:

* fetus
(FEE-tus) in humans is the developing offspring from nine weeks after conception until birth.
  • Amniocentesis (am-nee-o-sen-TEE-sis): In amniocentesis, a needle is passed through the mothers belly into her uterus* to collect some of the fluid in which the fetus lives. This fluid, called amniotic fluid, contains cells from the fetus.
  • Chorionic villus (kor-e-ON-ik VIL-us) sampling (CVS): CVS also involves collecting cells from the fetus with a needle. In this case, the cells are taken from the chorionic villi, which are structures in the uterus that are part of the placenta.
  • Percutaneous umbilical (per-ku-TAY-ne-us um-BIL-i-kal) blood sampling (PUBS): In PUBS, fetal blood is taken from the umbilical cord*.
* uterus
(U-ter-us), also called the womb, is the organ in a womans body in which a fertilized egg develops into a fetus.
* umbilical cord
(um-BIL-i-kal cord) is the flexible cord that connects a fetus at the navel with the placenta, the organ that allows for the exchange of oxygen, nutrients, and other substances between mother and fetus.

Genetic testing and counseling

Geneticists believe that each person probably carries about 5 to 10 defective recessive genes. Thus,

Inheritance Patterns of Some Genetic Diseases
Autosomal dominant Autosomal recessive X-linked dominant X-linked recessive Multiple genes
AchondroplasiaAlbinismDiabetes insipidusColor blindnessAlzheimers disease
Huntingtons diseaseCystic fibrosis(one form)HemophiliaSome cancers
NeurofibromatosisPhenylketonuria (PKU) Hunters syndrome(breast, colon, lung)
 Sickle-cell anemia Muscular dystrophyGout
 Tay-Sachs disease (Duchenne type)Rheumatoid arthritis

both potential parents may be worried about having a child with birth defects. If relatives have genetic disordersor if ethnic or other background factors increase the risk of certain genetic diseasesparents-to-be may worry even more.

Punnett Squares

Punnett squares often are used to visualize the chances of inheriting a particular gene. Using G for a healthy gene and g for an affected recessive gene, the Punnett Square shows which offspring are likely to inherit two healthy genes, which offspring are likely to be carriers of the gene, and which are likely to have the disorder caused by the defective gene.

Many medical centers now offer genetic testing and genetic counseling. Parents and relatives can be tested to determine whether they carry genes for a variety of disorders. Using this information, a genetic counselor can help couples calculate genetic risks realistically, and inform them about the options they may have to increase the likelihood of having a healthy child.

Ethical concerns

Increasingly, people will have the option to be tested to find out if they carry genes for genetic disorders. For example, women now can find out if their unborn children have certain genetic defects or if they themselves have genes that make them more likely to develop breast cancer. Already there is controversy about how this information should be used. Genetic testing can have far-reaching social, financial, and ethical effects. For example, a woman who thinks she will develop breast cancer might opt not to have children, or she might decide to have her breast tissue removed before cancer cells develop, or her insurance company might decide not to insure her because she is a high-risk client. With knowledge comes responsibility, and genetic testing surely will be at the forefront of debates about medical ethics in the twenty-first century.

See also

Albinism

Birth Defects

Breast Cancer

Color Blindness

Colorectal Cancer

Cystic Fibrosis

Down Syndrome

Dwarfism

Growth Disorders

Hemophilia

Huntington’s Disease

Muscular Dystrophy

Neurofibromatosis

Sickle-Cell Anemia

Tay-Sachs Disease

Turner Syndrome

Resources

Books

Baker, Catherine. Your Genes, Your Choices. Washington, DC: American Association for the Advancement of Science, 1997. A clear introduction to the ethical, legal, and social issues raised by genetic research. The full text of this book can be found on the associations website. http://www.aaas.org

Jackson, John F Genetics and You. Totowa, NJ: Humana Press, 1996. This book explains the basic principles of genetics, genetic counseling, and prenatal testing.

Organizations

Alliance of Genetic Support Groups, 4301 Connecticut Avenue Northwest, Number 404, Washington, DC 20008-2304. This national organization is an alliance of support groups for people who have or who are at risk for genetic disorders. Telephone 800-336-GENE http://www.geneticalliance.org

March of Dimes Birth Defects Foundation, 1275 Mamaroneck Avenue, White Plains, NY 10605. This large, national organization provides information about genetic birth defects. Telephone 888-MODIMES http://www.modimes.org

U.S. National Human Genome Research Institute, 31 Center Drive, Building 31, Room 4B09, MSC 2152, Bethesda, MD 20892. This government institute is home to the Human Genome Project, an international research effort aimed at mapping the human genome. http://www.nhgri.nih.gov

U.S. National Center for Biotechnology Information, National Library of Medicine, Building 38A, Room 8N805, Bethesda, MD 20894. This division of the U.S. National Library of Medicine provides detailed information about genes and genetic diseases. http://www.ncbi.nlm.nih.gov

World Health Organization (WHO), Avenue Appia 20, 1211 Geneva 27, Switzerland. The World Health Organization posts an extensive list of publications from its Human Genetics Programme at its website. http://www.who.int/ncd/hgn/hgn_pub.htm

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genetic diseases

genetic diseases Also known as inborn errors of metabolism. Diseases due to a single defective gene, with a characteristic pattern of inheritance in families. Many affect the ability to metabolize individual amino acids or carbohydrates and can be treated by dietary restriction. See also amino acid disorders; disaccharide intolerance.

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