Hereditary Diseases
Virtually all diseases involve genetic factors to some extent. Diseases that appear to result from simple mutations of single genes are often referred to as hereditary diseases, and they exhibit distinctive patterns of inheritance in families. There are approximately 6000 genetic diseases presently known.
Autosomal dominant genetic diseases are passed along from parent to child with an equal chance for sons and daughters to get the disease. This is sometimes referred to as vertical transmission because it is seen in each generation, usually without skipping. Normally there are two working copies of every gene in each individual. In the case of a dominant genetic disease, one copy of the gene is altered by mutation and the other copy is normal. The inheritance pattern seen in autosomal dominant diseases is that parents who show the trait pass the mutation on to half of their children. Children who do not have the trait will generally not pass the trait along to their children. Examples of autosomal dominant diseases include achondroplasia (a form of dwarfism), neurofibromatosis, and Huntington disease.
The inheritance pattern in autosomal recessive genetic diseases is distinctive in that the parents and other relatives of the person with the disease appear to be completely normal, while 25% of their brothers and sisters will share the same disease. This is sometimes called horizontal transmission because there is no expression seen in previous generations by the ancestors and relatives who carry the mutation. Rather, the mutation travels silently within the family and is expressed by siblings in a single generation. In order to get the disease, both parents must be carriers of the same mutated gene, and both must pass the mutation to the child in order for the child to get the disease. Examples of autosomal recessive disease include sickle cell anemia, cystic fibrosis, Tay-Sachs disease, and phenylketonuria.
X-linked recessive diseases are caused by genes on the X chromosome. Because males have only one X chromosome, they tend to express all mutations they inherit from their mother's X chromosome. Daughters receive an X chromosome from each parent and they, therefore, have a second copy of each gene to compensate for any mutations they might inherit. For this reason, the great majority of patients with X-linked recessive diseases are male. The inheritance pattern is characterized by clusters of diseased males who are related through apparently healthy female relatives. Examples of XLR diseases include hemophilia (types A and B), and Duchenne muscular dystrophy.
Heredity plays a role in virtually every disease, although sometimes there is interaction between genetic and nongenetic factors. Such diseases are termed multifactorial, or polygenic disorders, and their inheritance patterns can be quite complex. Most chronic illnesses in humans are multifactorial hereditary diseases. Examples include heart disease, diabetes, stroke, hypertension, cancer, and most forms of mental illness. As these complex patterns of inheritance become better understood, it is hoped that control of the nongenetic factors might help to reduce the burden of these common diseases.
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