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Understanding the meaning of a family history

How do I know if I have a family history of a disease? How do doctors define family history? And what should my family do about it? These questions are difficult for everyone. Doctors are often unsure of how to advise family members, who often worry that their loved ones may also develop the disease. The following discussion will explain what it means to have a family history of prostate cancer.

A family history of a disease means that one or more blood-related family members are affected with a certain disease. A family history of particular disease indicates that the disease is possibly associated with some aspect shared by the family members. Current thinking divides these factors into two categories -- nature and nurture. nurture means that our environment -- things like the food we eat, air we breathe, our daily activities -- have profound implications for our bodies, both in health and disease. Nature refers to the aspects of our health that are predetermined at birth, regardless of our environment. This refers to our genetic make-up. We have two copies of each gene -- one copy from our mother, and one copy from our father. There are many different genes within a population. This largely accounts for human diversity. It is thought that certain genes may make some individuals more or less likely to develop a certain disease.

Both nature and nurture influence our family history. This is because families often share the same environment -- shaped by such things as where we live, what we eat, our culture. Often these aspects are similar among family members. However, our genetic make-up is also shared within blood-related families. Since we receive half or our genes from our mother and half from our father, if one family member has a disease caused by a mutated gene (s), it is possible that other family members may laso have this disease. Between 5% and 10% of all cancers are thought to be caused by inherited mutated genes.

When scientists examine the link between our genes and our family history, they often create a diagram, or family tree. In this diagram, scientists use a symbol to represent each family member, and connect family members by lines, to create the family tree. Symbols connected by a horizontal line represent two married persons. Children are connected to their parents by a vertical line extending from the horizontal line between their two parents.

The different symbols used are presented in Figure 1.

There are several types of genetic inheritance patterns that cause diseases in humans. Most diseases with genetic causes do not follow simple patterns of inheritance; these types of diseases are caused by a variety of both nature and nurture factors. However, there are two simple types of genetic patterns that account for several diseases. An understanding of these two genetic patterns illustrates how genes influence the link between disease and family history. The first genetic pattern is known as a dominant genetic disorder. In this disorder, only one mutated gene copy (of the two copies) is required for an individual to have the disease. This means that if one parent with the disease (who carries one copy of the mutated gene) has children, each child has a 50 percent chance of inheriting the diseased gene, and a 50 percent chance of inheriting the normal gene from the affected parent (Figure 2). An example of a disease caused by a dominant mutation is Huntington's disease. Remember, though, when children have a 50 percent chance of inheriting a genetic disorder, it is not necessarily true that exactly 50 percent of children will have the genetic disorder.

Rather, this situation is similar to flipping a coin. Each time you flip the coin, there is a 50 percent chance it will turn up heads, and a 50 percent chance it will turn up tails. But, if you flip the coin, say twenty times, you will not necessarily get heads half of the time.

The second simple genetic pattern is called a recessive genetic disorder. To inherit a recessive genetic disorder, both copies of your disease gene must be mutated. If an individual has only one mutated copy and the other copy is normal, then he or she is said to be a carrier. When both parents are carriers, there is a 25 percent chance that their offspring will inherit both mutated genes (see Figure 3). An example of a disease caused by a recessive mutation is cystic fibrosis.

Despite our understanding of recessive and dominant genetic disorders, scientists often cannot explain why most diseases do not follow these simple patterns of inheritance. In some cases, a yet-undiscovered gene may be the cause of a disease. In other cases, the disease may result from complex interactions between one gene, multiple genes, and/or our environment. Our current understanding of the link between genetics and prostate cancer falls in this category - scientists have good evidence that genetics plays a role in developing the disease, but the exact genes are unknown. Current research, such as that conducted by the University of Michigan Prostate Cancer Genetics Project, is searching for the gene(s) that are involved in the development of prostate cancer.

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