Genetic diseases are disorders inherited from one's biological parents or acquired as a result of a spontaneous genetic change.
Genetic diseases are disorders that appear in individuals as a result of heredity, which is the passing of biological traits from parents to offspring. Individuals develop under the influence of a mix of genes * inherited from their biological parents. These genes, or small parts of chromosomes * , determine the architecture and activity of the entire body. They determine such visible characteristics as eye color, skin color, and height, as well as traits that cannot be seen, such as the risk 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 inherited from the mother and some from the father but who is never identical to either parent.
Individuals can inherit genetic disorders. This fact means that they are born with the disease even though some diseases may have no symptoms at first. In some instances, individuals do not inherit a genetic disease, but instead they develop a genetic disorder that occurs spontaneously when disease-causing mutations * occur during cell division * . Because these spontaneous diseases involve changes in the genes, they are also known as genetic disorders.
Some inherited genetic disorders, such as cystic fibrosis * and phenyl ketonuria * (PKU), result simply from the inheritance of genes that do not work properly. In other disorders, genetic and environmental factors work together to cause changes in otherwise normal genes. For example, some forms of radiation or toxic chemicals can cause cancer in people who are prone to be affected due to their genetic makeup.
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 successive generations in predictable patterns, and he began to outline the basic rules of inheritance. These rules are called Mendelian (men-DEL-ee-an) laws.
According to Mendelian laws, each person normally 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. An individual can have one of three combinations: two dominant genes (GG), two recessive genes (gg), or one of each (Gg). If a person has two dominant genes, the dominant trait will appear. If the individual has two recessive genes, the recessive trait will appear. If the individual has one dominant gene and one recessive gene, only the dominant trait will appear because it is the stronger of the two. In Mendel's peas, the trait for smooth peas was dominant over the trait for wrinkled peas. If a pea plant contains two genes for “smooth,” or one gene for “smooth” and one for “wrinkled,” the seed will be smooth. The pea is only wrinkled when it has two genes for “wrinkled.”
Of the 23 pairs of chromosomes in human cells, one pair is sex chromosomes, which determine a person's biological sex. Sex chromosomes are called X and Y chromosomes. Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The reproductive cells (egg 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 the sex of the offspring. Inherited genetic disorders that are carried on the sex chromosomes are referred to as X-linked or sex-linked. Sex-linked disorders usually affect males more often than females, because a female has two copies of the X chromosome, each of which carries a set of genes. In contrast, a male has only one X chromosome, so he has only one set of these genes. If his X chromosome is damaged or defective, he has no normal copy to override or mask the defective one. Depending on the specific problem with the gene, the result can be an X-linked disorder.
Besides the sex chromosomes, human cells have 22 other pairs of chromosomes. These are called autosomes (AW-to-somes), or non-sex chromosomes, and disorders carried on these chromosomes, which are numbered 1 through 22, are referred to as autosomal (AW-to-SOME-al). In general, autosomal disorders are likely to affect males and females equally.
Most genetic disorders are caused by defective genes on the autosomes. If a single gene is responsible, then the following rules of inheritance usually apply. Exceptions exist, but these rules are useful guidelines for understanding autosomal dominant inheritance:
In an autosomal recessive disorder, the following applies:
More than 150 disease traits are carried on the X chromosome, but X-linked dominant disorders are rare. In an X-linked recessive disorder, the following applies:
Other genetic disorders result from extra or missing chromosomes. In Down syndrome * , a person has three copies of chromosome 21 rather than the usual two copies. In 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 * .
Sometimes, a child may be born with a dominant inherited disorder even though neither parent has the disorder. The cause is usually a spontaneous (or new) mutation in a gene or genes. 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-KONdro-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 earlier.
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 scientists described the structure of DNA. From the 1980s into the 21st century, scientists' understanding of genes and how they work grew at an incredibly rapid pace. They identified many disease-causing genes, opening the door to research on ways to fix genetic defects. This field of science is referred to as gene therapy.
A number of treatments for genetic disorders are available. For some disorders, the treatment is a special diet 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 and essentially short-circuiting the disease. A third kind of treatment called gene therapy actually replaces defective genetic material with normal genetic material inside the cells. Researchers are considering a variety of methods to perform this procedure. These include the use of microscopic “bullets” consisting of genetic material and viruses to deliver normal genes to cells.
Medical professionals can test a fetus *
Geneticists believe that each person probably carries about 5 to 10 defective recessive genes. As a result, potential parents may be worried about having a child with birth defects. If blood relatives have genetic disorders—or if ethnic or other background factors increase the risk of certain genetic diseases—parents-to-be may be even more concerned. For this reason, some adults seek genetic testing of themselves or their relatives to determine whether they are carriers and whether their future children are at risk of certain genetic disorders. Medical experts recommend that genetic testing be followed by counseling, which advises clients of the nature of the disorders, their options in family planning, and any steps they can take to increase the likelihood of having a healthy child.
People in the 21st century are increasingly likely to have the option to be tested to find out whether they carry genes for genetic disorders. For example, women can find out whether their unborn children have certain genetic defects or whether they themselves have genes that make them more likely to develop such disorders as breast cancer. Advances in testing techniques have led to 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 may decide against having children or decide to have her breast tissue removed before cancer cells develop. With knowledge comes responsibility, and genetic testing surely will be at the forefront of debates about medical ethics for many years to come.
See also Albinism • Alzheimer's Disease • Anemia, Bleeding, and Clotting • Birth Defects: Overview • Breast Cancer • Charcot-Marie-Tooth Disease • Color Blindness • Cystic Fibrosis • Down Syndrome • Familial Mediterranean Fever • Fanconi Anemia • Gout • Huntington Disease • Intellectual Disability • Marfan Syndrome • Microcephaly • Muscular Dystrophy • Neurofibromatosis • Phenylketonuria (PKU) • Porphyria • Sickle Cell Anemia • Tay-Sachs Disease • Thalassemia • Turner Syndrome
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Genetic Alliance. 4301 Connecticut Ave. NW, Suite 404, Washington, DC 20008-2369. Telephone: 202-966-5557. Website: http://www.geneticalliance.org (accessed April 6, 2016).
March of Dimes. 1275 Mamaroneck Ave., White Plains, NY 10605. Telephone: 914-997-4488. Website: http://www.marchofdimes.org (accessed April 6, 2016).
National Center for Biotechnology Information, National Library of Medicine. 8600 Rockville Pike, Bethesda, MD 20894. Toll-free: 888-346-3656. Website: http://www.ncbi.nlm.nih.gov (accessed April 6, 2016).
National Human Genome Research Institute. 9000 Rockville Pike, Bethesda, MD 20892. Telephone: 301-496-4000. Website: http://www.genome.gov (accessed April 6, 2016).
World Health Organization. Avenue Appia 20 1211, Geneva 27, Switzerland. Telephone: 41-22-791-21-11. Website: http://www.who.int/en/ (accessed April 6, 2016).
* genes (JEENS) are chemical structures composed of deoxyribonucleic acid (DNA) that help determine a person's body structure and physical characteristics. Inherited from a person's parents, genes are contained in the chromosomes found in the body's cells.
* chromosomes (KRO-mo-somz) are threadlike chemical structures inside cells on which the genes are located. There are 46 (23 pairs) chromosomes in normal human cells. Genes on the X and Y chromosomes (known as the sex chromosomes) help determine whether a person is male or female. Females have two X chromosomes; males have one X and one Y chromosome.
* 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.
* cystic fibrosis (SIS-tik fy-BRO-sis) is a disease that causes the body to produce thick mucus that clogs passages in many of the body's organs, including the lungs.
* phenylketonuria (FEN-ul-keton-U-ree-a), or PKU, is a genetic disorder of body chemistry that, if left untreated, causes intellectual disability.
* genetics (juh-NEH-tiks) is the branch of science that deals with heredity and the ways in which genes control the development and maintenance of organisms.
* carrier is a person who has in his or her body a bacterium or virus or gene for a disease that can be transmitted to other people without the carrier becoming sick or developing the disease.
* Down syndrome is a genetic disorder that can cause intellectual disability, shortness, and distinctive facial characteristics, as well as many other features.
* cri du chat (KREE-doo-SHA), French for cat's cry, is a genetic disorder that can cause intellectual disability, 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.
* embryo (EM-bree-o), in humans, is the developing organism from the end of the second week after fertilization to the end of the eighth week.
* fetus (FEE-tus) is the term for an unborn human after it is an embryo, from nine weeks after fertilization until childbirth.
* uterus (YOO-teh-rus) is the muscular, pear-shaped internal organ in a woman where a baby develops until birth.
* umbilical cord (um-BIH-lih-kul) is the flexible cord that connects a baby to the placenta, the organ that unites the unborn child to the mother's uterus, the organ in which the baby develops.