American Medical Association Family Medical Guide - American Medical Association [659]
13
Genetic Disorders
Like eye color and hair color, some disorders can be transmitted from one or both parents to a child. Sometimes, normal genes change (mutate) as cells divide, or genes in a parent’s egg or sperm are altered by environmental factors such as radiation to produce a genetic disorder in a child whose parents are both healthy. The child, who carries the genetic mutation in each of his or her cells, can then pass the mutated gene to the next generation.
Genetic disorders fall into three general categories—chromosome abnormalities, single-gene disorders, and disorders that result from an interaction of numerous genes and the environment. Chromosome abnormalities result from defects in the number or structure of chromosomes (structures that contain numerous genes). Single-gene disorders result from a single defective gene or pair of genes, are usually inherited, and are likely to recur in a family.
The way in which single-gene disorders are inherited depends on whether the defective gene is a dominant gene or a recessive gene on an autosome (one of the 22 chromosomes that are the same in males and females) or a gene on the X sex chromosome. To be affected by an autosomal dominant disorder, a person needs to receive only one copy of a dominant disease gene from one parent. Autosomal recessive disorders usually affect a person only if he or she has received two copies of the recessive disease gene, one from each parent. A person who has only one copy of a defective recessive gene is called a carrier. X-linked, or sex-linked, disorders are usually transmitted from mothers (who are unaffected carriers of the defective gene on their X chromosome) to sons, who are affected (because they don’t have another X chromosome, as females do, to override the effects of the defective gene).
The most common chronic disorders—including heart disease, cancer, asthma, osteoporosis, and type 2 diabetes—usually result from the interaction of several genes with environmental influences such as diet and smoking. People who inherit genes that can make them susceptible to developing a particular disorder are not destined to get the disease unless they expose their body to the stresses that trigger the susceptibility genes. For example, people who inherit genes that make them susceptible to developing heart disease will not necessarily develop heart disease unless they eat a high-fat diet, smoke cigarettes, fail to exercise, or are overweight. Conversely, not having these susceptibility genes does not guarantee that a person will not develop a disease if he or she smokes or engages in other unhealthy habits.
If you have relatives who have had illnesses such as cancer or heart disease at a young age or who have died from such an illness at a young age, their illnesses were likely to have a strong genetic component. In this case, you might benefit from seeing a genetic counselor, who can help you understand your personal health risks and the steps you might take to reduce these risks. A genetic counselor can also explain how your family health history (see page 131) might affect your children.
Genetic disorders can affect different organ systems. Hemophilia, sickle cell disease, and thalassemia affect the blood; muscular dystrophy affects the muscles; cystic fibrosis affects the lungs; and Huntington disease and Tay-Sachs disease affect the brain and nervous system. Inborn errors of metabolism, such as Gaucher disease and galactosemia, are imbalances in the body’s chemistry caused by a defective gene or pair of genes that a child inherits from one or both parents.
The severity of genetic disorders can vary from disorder to disorder, and the same disorder can differ in severity from one person to another. For example, Duchenne muscular dystrophy is always fatal during adolescence or early adulthood, while hemophilia can be treated successfully enough to enable an affected person to lead a relatively full life. Sickle cell disease and Marfan syndrome can be mild in one person and severe in another.