Genetic Disorders - Common Genetically Inheriteddiseases, Alzheimer's Disease, Cancer, Cystic Fibrosis, Diabetes, Huntington's Disease
genes chromosome environmental diseases
We could wish that … life-histories were found in every family, showing the health and diseases of its different members. We might thus in time find evidences of pathological connections and morbid liabilities not now suspected.
—William Gull, 1896
It has long been known that heredity affects health. Genetics, the study of single genes and their effects on the body and mind, explains how and why certain traits such as hair color and blood types run in families. Genomics, a discipline that is only about two decades old, is the study of more than single genes; it considers the functions and interactions of all the genes in the genome. In terms of health and disease, genomics has a broader and more promising range than genetics. The science of genomics relies on knowledge of and access to the entire genome and applies to common conditions, such as breast and colorectal cancer, Parkinson's disease, and Alzheimer's disease. It also has a role in infectious diseases once believed to be entirely environmentally caused such as human immunodeficiency virus (HIV, which is the virus that causes acquired immune deficiency syndrome [AIDS]) infection and tuberculosis. Like most diseases, these frequently occurring disorders are due to the interactions of multiple genes and environmental factors. Genetic variations in these disorders may have a protective or a causative role in the expression of diseases.
It is commonly accepted that diseases fall into one of three broad categories: those few that are primarily genetic in origin; those that are largely attributable to environmental causes; and those—the majority of conditions—in which genetics and environmental factors make comparable, though not necessarily equal, contributions. As understanding in genomics advances and scientists identify genes involved in more diseases, the distinctions between these three classes of disorders is diminishing. This chapter considers some of the disorders believed to be predominantly genetic in origin and some that are the result of genes acted on by environmental factors.
There are two types of genes: dominant and recessive. When a dominant gene is passed on to offspring, the feature or trait it determines will appear regardless of the characteristics of the corresponding gene on the chromosome inherited from the other parent. If the gene is recessive, the feature it determines will not show up in the offspring unless both the parents' chromosomes contain the recessive gene for that characteristic. Similarly, among diseases and conditions primarily attributable to a gene or genes, there are autosomal dominant disorders and autosomal recessive disorders.
Another way to characterize genetic disorders is by their pattern of inheritance, as single gene, multifactorial, chromosomal, or mitochondrial. Single-gene disorders (also called Mendelian or monogenic) are caused by mutations in the deoxyribonucleic acid (DNA) sequence of one gene. Since genes code for proteins, when a gene is mutated so that its protein product can no longer carry out its normal function, it may produce a disorder. There are more than 6,000 known single-gene disorders, which occur in about one in every 200 births. Examples are cystic fibrosis, sickle-cell anemia, Huntington's disease, and hereditary hemochromatosis (a disorder in which the body absorbs too much iron from food; rather than the excess iron being excreted, it is stored throughout the body, and iron deposits damage the pancreas, liver, skin, and other tissues). Figure 5.1 shows the cystic fibrosis gene and its location on chromosome 7; Figure 5.2 shows the sickle-cell anemia gene found on chromosome 11; and Figure 5.3 shows the hereditary hemochromatosis gene located on chromosome 6. Single-gene disorders are the result of either autosomal dominant, autosomal recessive, or X-linked inheritance.
Multifactorial or polygenic disorders result from a complex combination of environmental factors and mutations in multiple genes. For example, different
FIGURE 5.1
The cystic fibrosis gene
Chromosomal disorders are produced by abnormalities in chromosome structure, missing or extra copies of chromosomes, or errors such as translocations (movement of a chromosome section from one chromosome to another). Down's syndrome or trisomy 21 is a chromosomal disorder that results when an individual has an extra copy, or a total of three copies, of chromosome 21. Mitochondrial disorders result from mutations in the nonchromosomal DNA of mitochondria, which are organelles involved in cellular respiration. Compared with the three other patterns of inheritance, mitochondrial disorders occur infrequently.
There are significant differences between the nineteenth-century germ theory of disease and the twenty-first-century genomic theory of disease. By
FIGURE 5.2
The sickle cell anemia gene
At the dawn of the twenty-first century, the possibility of preventing and changing genetic legacies appears within reach of modern medical science. Genomic medicine predicts the risk of disease in the individual, whether highly probable, as in the case of some of the well-established single-gene disorders, or in terms of an increased susceptibility likely to be influenced by environmental factors.
FIGURE 5.3
The hemochromatosis gene
The enzyme phenylalanine hydroxylase converts the amino
acid phenylalanine to tyrosine
Additional Topics
Genetic Disorders - Common Genetically Inheriteddiseases
Although many diseases, disorders, and conditions are termed "genetic," classifying a disease as genetic simply means that there is an identified genetic component to either its origin or its expression. Many medical geneticists contend that the majority of diseases cannot be classified as strictly genetic or environmental. The phenotype of genetic diseases can sometimes be modified,…
Genetic Disorders - Alzheimer's Disease
Alzheimer's disease (AD) is a progressive, degenerative disease that affects the brain and results in severely impaired memory, thinking, and behavior. It is the fourth leading cause of death in adults, and the incidence of the disease rises with age. AD affects an estimated four million American adults and is the most common form of dementia, or loss of intellectual function. The U.S. Depa…
Genetic Disorders - Cancer
Cancer is a large group of diseases characterized by uncontrolled cell division and the growth and spread of abnormal cells. These cells may grow into masses of tissue called tumors. Tumors composed of cells that are not cancerous are called benign tumors. Tumors consisting of cancer cells are called malignant tumors. The dangerous aspect of cancer is that cancer cells invade and destroy normal ti…
Genetic Disorders - Cystic Fibrosis
Cystic fibrosis (CF) is the most common inherited fatal disease of children and young adults in the United States. It occurs in about one in every 3,000 Caucasian births, one in 15,000 African-Americans and one in 31,000 Asian-Americans. According to the NIH (http://www.genome.gov/10001213), approximately 30,000 young people had the disease in 2005; their median (half above and half below) life sp…
Genetic Disorders - Diabetes
Diabetes is a disease that affects the body's use of food, causing blood glucose (sugar levels in the blood) to become too high. Normally, the body converts sugars, fats, starches, and proteins into a form of sugar called glucose. The blood then carries glucose to all the cells throughout the body. In the cells, with the help of the hormone insulin, which facilitates the entry of glucose in…
Genetic Disorders - Huntington's Disease
Named for an American physician, George Sumner Huntington (1850–1916), Huntington's disease (HD), or Huntington's chorea, is an inherited, progressive brain disorder. It causes the degeneration of cells in the basal ganglia, a pair of nerve clusters deep in the brain that affect both the body and the mind. HD is caused by a single dominant gene that affects men and women of al…
Genetic Disorders - Muscular Dystrophy
Muscular dystrophy (MD) is a term that applies to a group of more than thirty types of hereditary muscle-destroying disorders. Approximately one million Americans are affected by one of the forms of MD. Each variant of the disease is caused by defects in the genes that play important roles in the growth and development of muscles. Duchenne muscular dystrophy (DMD) is one of the most frequently occ…
Genetic Disorders - Phenylketonuria
Phenylketonuria (PKU) is an example of a disorder caused by a gene-environment interaction. As a result of the defect, the affected individual is unable to convert phenylalanine into tyrosine. Phenylalanine in the body accumulates in the blood and can reach toxic levels. (See Figure 5.4.) This toxicity may impair brain and nerve development and result in mental retardation, organ damage, unusual p…
Genetic Disorders - Sickle-cell Disease
Sickle-cell disease (SCD) is a group of hereditary diseases, including sickle-cell anemia (SCA) and sickle B-thalassemia, in which the red blood cells contain an abnormal hemoglobin, termed hemoglobin S (HbS). HbS FIGURE 5.19 Sickle cell anemia SOURCE: "Sickle Cell Anemia," in Genes and Disease, National Institutes of Health, National Center for Biotechnology Information, 2004, …
Genetic Disorders - Tay-sachs Disease
Tay-Sachs disease (TSD) is caused by mutations in the HEXA gene, located on the long arm of chromosome 15. (See Figure 5.21.) It is a fatal genetic disorder in children that causes the progressive destruction of the central nervous system. It is caused by the absence of an important enzyme called hexosaminidase A (hex-A). Without hex-A, a fatty substance called GM2 ganglioside builds up abnormally…
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