Understanding How Genes Impact Disease

What Are Genes?

Genes play a huge role in who we are. Be it from the color of our eyes to the seasonal allergies that make us sneeze. Our parent's genes predispose us to behave, appear, and feel in ways that are sometimes beyond our control. And because our genetic makeup is passed from generation to generation, there is sometimes the risk that we will inherit negative characteristics as well. Sometimes these negative traits come in the form of a genetic disorder or disease.

Aside from the everyday cold or flu, many illnesses do have a genetic component. Our genes may not necessarily cause an illness, but they might influence its development, or make it more likely that we will develop a disease. Our genes also regulate and modulate our body's response to an illness or a medication. Genes are units of DNA which produce individual proteins that keep our body's machinery working. Each of us has 30,000-40,000 genes in each one of our cells, and half of these are inherited from each parent. Remarkably, we differ from one another only by 0.1 percent of our DNA! But a defect, or mutation, in a single gene can cause a serious (or worse, fatal) genetic disorder that can be transmitted through a family.

Dominant Genes

We inherit our genes in pairs, one from each parent. If one of the gene pair has a stronger effect than its mate, it is called a dominant gene. If there is a defect in that dominant gene, then the parent likely has a genetic disorder, and there is a 50 percent chance that this affected parent will pass this mutated gene onto each of her offspring. There are a few thousand dominant genetic disorders. Examples include, Huntington's disease (a degenerative disease of the brain), Adult Polycystic kidney disease, Neurofibromatosis, and Marfan's syndrome.

If one parent has a defective dominant gene:

• There is a 50 percent risk that each child will inherit the gene
• These is an equal chance of transmitting to sons and daughters and of receiving the gene from either parent
• There will be a total absence of the disease from those who do not inherited the harmful gene
• The genetic disorder will appear in every generation (even though signs may be subtle).

The idea that a disease skips a generation is mostly untrue. Most likely, the disease manifested itself in such a way that it was missed because the symptoms were so slight, not looked for, or not recognized. From time to time, a new dominant gene disease will occur for the first time in a family. Afterwards, the person affected, with his new mutation, will transmit the disorder to half of his children. For some of these disorders, especially those that affect the nervous system, the disease will present itself earlier and earlier over each succeeding generation. The result can be as dramatic as a child less than five years of age presenting symptoms and signs of Huntington's disease.

Recessive Genes

When defects occur in each of the matching genes from both parents, a recessive genetic disease may occur. Examples of recessive genetic diseases include, cystic fibrosis, sickle cell disease, and Tay-Sachs disease. Typical characteristics of recessive inheritance include:

• These is an equal chance of transmitting to sons and daughters and of receiving the gene from either parent
• There is a one in four risk in every pregnancy that the child will be affected.
• Parents who may be related (e.g., cousins)

Usually (but not always) neither parent nor other relatives (other than the child's siblings) are affected by the gene. Also, some recessive conditions are higher amongst certain ethnic groups as the members of that group may share, as cousins do, some of their genes. There are well-recognized recessively inherited disorders in every ethnic or racial group, and most of these do have a bearing on pregnancy risks.