Genetic disorders are conditions that have some origin in an individual's genetic make-up. Many of these disorders are inherited and are governed by the same genetic rules that determine dimples and red hair. However, some genetic disorders—such as Down syndrome, characterized by heart malformation, poor muscle tone, and a flattened face—result from a spontaneous mutation (gene change) that takes place during embryonic (earliest life) development.
Genetic disorders can be classified according to the way in which they develop. If the disorder is transmitted by genes inherited from only one parent, it is said to be an autosomal dominant disorder. The term autosome applies to any of the 22 chromosomes that are identical in human males and females. (Chromosomes are structures that organize genetic information in the nucleus of cells.) By contrast, disorders that can be inherited only by the transmission of genes from both parents is called an autosomal recessive disorder.
Other genetic disorders are associated with the X (female) or Y (male) chromosome and are called sex-linked disorders because the X and Y chromosomes are related to sexual characteristics in humans. Finally, the development of some genetic disorders involves environmental factors, factors present outside the organism itself. Such disorders are known as multifactorial genetic disorders.
Principles of genetic inheritance
Genetic information in humans is stored in units known as genes located on large complex molecules called chromosomes. A vast range of human characteristics, from eye and hair color to musical and literary talents, are controlled by genes. To say that a person has red hair color, for example, is simply to say that that person's body contains genes that tell hair cells how to make red hair.
Reproduction in humans occurs when a sperm cell from a male penetrates and fertilizes an egg cell from a female. The fertilized egg cell, called a zygote, contains genes from both parents. For example, the zygote will contain two genes that control hair color, one gene from the mother and one gene from the father.
In some cases, both genes carry the same message. For example, the zygote might contain two genes that act as a kind of code that tells a cell to make red hair, one from each parent. In that case, the child will be born with red hair.
Words to Know
Chromosomes: Structures that organize genetic information in the nucleus of cells.
Dominant trait: A trait that can manifest (be expressed) when inherited from one parent.
Gene: A section of a chromosome that carries instructions for the formation, functioning, and transmission of specific traits from one generation to another.
Multifactorial trait: A trait that results from both genetic and environmental influences.
Proteins: Large molecules that are essential to the structure and functioning of all living cells.
Recessive trait: A trait that is expressed in offspring only when identical genes for the trait are inherited from both parents.
Sex-linked disorder: A disorder that generally affects only one sex (male or female).
In other cases, two genes may carry different messages. The zygote might, for instance, carry a gene for red hair from the mother and for brown hair from the father. In such cases, one gene is dominant and the other recessive. As these terms suggest, one gene will "win out" over the other and determine the offspring's hair color. In this example, the gene for brown hair is dominant over the gene for red hair, and the offspring will have brown hair.
Dominant genetic disorders
If one parent has an autosomal dominant disorder, then offspring have a 50 percent chance of inheriting that disease. Approximately 2,000 autosomal dominant disorders (ADDs) have been identified. These disorders have effects that range from inconvenience to death. ADDs include Huntington's disorder, polydactyly (extra toes or fingers), Marfan's syndrome (extra long limbs), achondroplasia (a type of dwarfism), some forms of glaucoma (a vision disorder), and hypercholesterolemia (high blood cholesterol).
ADDs may occur early or late in life. People with ADDs that are diagnosed at older ages are faced with very special problems. They may already have had children of their own and transmitted the genetic trait that caused their disorder to their offspring.
Huntington's disease (also known as Huntington's chorea) is an example of an ADD that is typically diagnosed relatively late in life. The
disorder is characterized by progressive involuntary, rapid, jerky motions and mental deterioration. It usually appears in affected individuals between the ages of 30 and 50, and leads to dementia and eventual death in about 15 years.
Marfan's syndrome, also called arachnodactyly, is an ADD characterized by long, thin arms, legs, and fingers. People with Marfan's syndrome also tend to be stoop-shouldered and have a bluish tint to their eyeballs. In addition, these individuals have a high incidence of eye and heart problems. Abraham Lincoln is believed to have had Marfan's syndrome.
Recessive genetic disorders
Recessive genetic disorders (RGD) are caused when both parents supply a recessive gene to their offspring. The probability of such an event's occurring is 25 percent each time the parents conceive. About 1,000 confirmed RGDs exist. Some of the better known examples of the
condition include cystic fibrosis, sickle-cell anemia, Tay-Sachs disease, galactosemia, phenylketonuria (PKU), adenosine deaminase deficiency, growth hormone deficiency, Werner's syndrome (juvenile muscular dystrophy), albinism (lack of skin pigment), and autism.
Some RGDs tend to affect people of one particular ethnic background at a higher rate than the rest of the population. Three such RGDs are cystic fibrosis, sickle-cell anemia, and Tay-Sachs disease. Cystic fibrosis is one of the most common autosomal recessive diseases in Caucasian children in the United States. About 5 percent of Caucasians carry this recessive gene. Cystic fibrosis is characterized by excessive secretion of an unusually thick mucus that clogs respiratory ducts and collects in lungs and other body areas. Cystic fibrosis patients usually die before the age of 20, although some individuals live to the age of 30.
Sickle-cell anemia occurs with an unusually high incidence among the world's black and Hispanic populations. However, some cases also occur in Italian, Greek, Arabian, Maltese, southern Asian, and Turkish people. About 1 in 12 blacks carry the gene for this disorder. Sickle-cell anemia is caused by mutations in the genes responsible for the production of hemoglobin. (Hemoglobin is the compound that carries oxygen in red blood cells to tissues and organs throughout the body.) Sickle-cell anemia patients have red blood cells that live only a fraction of the normal life span of 120 days. The abnormal blood cells have a sickled appearance, which led to the disease's name. Sickle-cell patients also die early, before the age of 30.
The Tay-Sachs gene is carried by 1 in 30 Ashkenazi Jews. Children born with Tay-Sachs disorder seem normal for the first 5 months of their lives. But afterwards, they begin to express symptoms of the disorder. Eventually, the condition leads to blindness and death before the age of four.
Galactosemia and PKU are examples of metabolic RGDs. A metabolic RGD is one in which a person's body is unable to carry out functions that are normal and essential to the body. For example, people with galactosemia lack an enzyme (chemical) needed to metabolize (break down) galactose, a sugar found in milk. If people with galactosemia do not avoid normal milk, mental retardation will eventually develop. People with PKU have a similar problem. They lack an enzyme needed to convert the amino acid phenylalanine to the amino acid tyrosine. The build-up of phenylalanine in the body leads to severe mental retardation.
Adenosine deaminase deficiency is one of few "curable" genetic diseases. It is caused by a mutation in a single gene essential to normal development of the immune system. Bone marrow transplants have been found to be of some value to patients. In addition, gene therapy has been successful at replacing these patients' defective gene with a copy of a correct gene that enables their immune system to function effectively.
Is the traffic light red or green? Most humans have the ability to distinguish the color we call red from the color we call green. But some people cannot. Such people are said to be color-blind. Color blindness is a defect in vision that makes it difficult or impossible for a person to distinguish between or among certain colors.
Color-blindness is usually passed on genetically, and is more common in men than in women. About 6 percent of all men and roughly one-tenth of that many women inherit the condition. Individuals also can acquire the condition through various eye diseases. There is no treatment for color blindness.
The most common form of color-blindness involves the inability to distinguish reds from greens. A less common condition involves the inability to distinguish green from yellow.
Color blindness is caused by a lack of pigment in the retina of the eye. Normally, the retina contains molecules capable of detecting every color in the spectrum. However, if some of these molecules are not present, the various colors in the spectrum can not be distinguished from each other, and the person is color-blind.
Color blindness is a sex-linked characteristic. The gene involved in the disorder occurs only on the X chromosome, which is passed to the child by the mother. The Y chromosome, which is passed to the child by the father, does not carry the defective gene. As a result, children inherit color blindness only from their mothers.
Sex-linked genetic disorders
Sex-linked genetic disorders (XLGDs) can be either dominant or recessive. Dominant XLGDs affect females, are usually fatal, and cause severe disorders in males who survive. A high percentage of male embryos with dominant XLGD spontaneously abort early in a pregnancy. Dominant XLGD's include conditions such as Albright's hereditary osteodystrophy (seizures, mental retardation, stunted growth), Goltz's syndrome (mental retardation), cylindromatosis (deafness and upper body tumors), oral-facial-digital syndrome (no teeth, cleft tongue, some mental retardation), and incontinentia pigmenti (abnormal swirled skin pigmentation).
Recessive XLGDs are passed to sons through their mothers. Major XLGDs include severe combined immune deficiency syndrome (SCID), color blindness, hemophilia, Duchenne's muscular dystrophy (DMD), some spinal ataxias, and Lesch-Nyhan syndrome. Roughly one-third of these XLGDs result from a spontaneous mutation. Of these disorders, color blindness is the least harmful.
Hemophilia is an example of a serious XLGD. This disorder is caused by the absence of a protein responsible for the clotting of blood. Lacking this protein, a person with hemophilia may easily bleed to death from simple cuts and injuries that would be of little danger to the average person. Hemophilia A is the most severe form of this disease, and is characterized by extreme bleeding. It affects males primarily, although it has been known to occur in females. The disorder has often been associated with royalty. England's Queen Victoria was a carrier whose descendants became rulers in several European countries.
Other usually fatal XLGDs affect the immune, muscular, and nervous systems. SCID, for example, is a disorder affecting the immune system. It is characterized by a very poor ability to combat infection. One way to treat patients with SCID is to completely enclose them in a large plastic bubble that protects them from germs present in the air. The only known cure for SCID involves a bone marrow transplant from a close relative.
DMD afflicts young boys and is apparent by age three or four. It is characterized by wasting leg and pelvic muscles. Patients with DMD are usually wheelchair-bound by the age of 12, and die before the age of 20, often as the result of heart problems.
Multifactorial genetic disorders
Scientists often find it difficult to determine the relative role of heredity and environment in certain medical disorders. One way to answer this question is with statistical and twin studies. Identical and fraternal twins who have been raised in different and identical homes are evaluated for these MFGDs. If fraternal twins have a higher than normal incidence of a disorder and identical twins show an even higher rate of the disorder, then genetic inheritance is believed to contribute to development of the disorder.
Among the most likely candidates for multifactorial genetic disorders are certain medical conditions associated with diet and metabolism, such as obesity, diabetes, alcoholism, rickets, and high blood pressure; some infectious diseases, such as measles, scarlet fever, and tuberculosis; schizophrenia and some other psychological illnesses; club foot and cleft lip; and various forms of cancer.
The tendency of some people to be more susceptible to a particular MFGD and not another is characteristic of human genetics. All healthy humans have a similar body form with very similar physiological functions. Still, it is easy to see tremendous human diversity that results from a diverse gene pool. This diversity explains why certain groups of people with similar kinds of genes are more prone to some disorders, whereas others have resistance to the same disorders. This diversity protects the human race from being wiped out by a single kind of medical problem.