Genetic Seeds of Disease

Some of the illnesses that afflict man have their origins in the physical equipment he inherited from his parents- in the unique set of genes with which he was born. Exactly how many disorders are thus inescapably built into human beings not even the modern geneticist knows for sure. Both heredity and environment are involved to some extent in all disease. In some cases, environment factors obviously play the critical role. A sufficiently large dose of barbiturates will kill anyone, regardless of his physical inheritance. But geneticist now know that each person's success in fighting off infections and other threats to health depends partly on his individual genetic endowment. Furthermore, many rare and strange diseases are definitely inherited.

The genes' role in the body's adaptation to the environment is illustrated by the reactions of different types of people to sunlight. Those with fair skin must take care not to be badly burned by a day at the beach. Darker-skinned people- who have inherited a better capacity to produce the pigment involved in tanning- adapt to sunlight with less skin damage. As one consequence, people with fair skins are more likely than brunets to develop skin cancer. Thus, although the cancer itself may not be inherited, certain people may be more vulnerable to it because of their genes.

Genes are probably the most intricate bundles of information known. They determine the nature of every cell in every living organism, establishing its species, sex and the general pattern of its individual characterictics. Humans, who are composed of trillions of highly specialized cell, have many thousands of genes. They are gathered in 46 curiously shaped agglomerations of matter called chromosomes, which come in pairs- one member of each pair from each parent. The basic genetic materials accomplishes its many tasks by duplicating itself completely in every new cell grown by the body; fron headquarters in the cell nucleus it then directs the cell's development.

Genes and environment are so intertwined in their effects upon development that a geneticist's hardest task is often to unravel what is genetic and what is not. A baby with a perfectly sound genetic inheritance, for example, may be born with a congenital defect- that is, a defect present at birth- because he was damaged by his environment. This damage may have been caused by anything from virus in his mother's bloodstream to an injury during delivery. Even certain diseases that seemed to run in families have turned out to be caused by enviroment. It had been known for a long time, for instance, that a large percentage of the females of certain strains of mice developed mammary cancer. The disease seemed definitely hereditary- until it was discovered that the milk of mice from these particular strains contained a cancer-inducing virus. By letting newborn mice of these strains suckle on mice of another strain, experimenters succeeded in ending this apparently hereditary trait.

Even when a disease is genetic, the geneticist may find it impossible to predict which apparently normal member of a family will develop it. One genetic disease, Hungtington's chorea, a progressive degeneration of the nervous system ending in death, may not show its first symptoms- involuntary jerking movements of the body and limbs- until a person is past middle age. If the victim has children, the statistical chances are that half of them will also be afflicted, but there is no way to predict who or when, for the disease may strike at any age.