Editor's Note: You can hardly turn on the news or pick up a magazine these days without seeing something on genes for diseases and tests for these genes. But, there's a lot of confusion about what gene defects (sometimes called "mutations") really are and what genetic tests can and can't tell us about ourselves and our children. Following are answers to frequently asked questions about genetics and genetic testing.

A.  Many people picture genes as beads on a string, but they're actually closer to being strings themselves strings of chemicals that make up DNA (deoxyribonucleic acid). These strings are arranged along structures known as chromosomes.

Chromosomes come in pairs. In humans, every cell has 22 pairs of "autosomes," chromosomes numbered 1 through 22. Each cell in each person also has two more chromosomes in females, a pair of X chromosomes, and in males, an X and a Y chromosome. Humans have 46 chromosomes. (Each animal has its own number.)

Most cells make exact copies of themselves when body tissues are developing, growing or being repaired. During this copying process, all the chromosomes in a cell duplicate themselves, line up in the center of the cell and then divide, forming two identical "daughter" cells.

But there's an important exception. Cells that make sperm and egg cells reduce their chromosome number by half as they replicate. The reason for this "reduction division" is that, when a sperm cell meets an egg cell to conceive a child, the new cell has to start with the right number of chromosomes for a human being 46.

Something else of great importance happens during reduction division. Cells that make sperm and egg cells shuffle their DNA before they divide, so that each of four daughter cells they produce gets a slightly different combination of DNA.

When a sperm cell combines with an egg cell, it contributes its DNA, which has been shuffled so that it's slightly different from the DNA in all the other cells of that person's body. Meanwhile, the egg cell contributes its DNA, which has gone through the same process.

Reduction division and reshuffling (called "recombination") explain why siblings (except for identical twins) each have unique combinations of traits from both parents.

A.  Every baby starts from two reproductive cells, a father's sperm cell and a mother's egg cell. Each of these contributes one of each autosome and either an X or a Y chromosome. A child gets 44 paired autosomes, 22 from each parent. A girl also gets an X from each parent, a boy an X from the mother and a Y from the father.

Sometimes, one or more of the chromosomes inherited from either parent carries a gene that has changes in its chemical makeup. These changes, called "mutations" when they're linked with disorders and "polymorphisms" when they aren't, can involve missing pieces of DNA, added pieces of DNA or substitutions of one DNA part for another.

Whether or not mutations cause a disease in a child depends on many factors, including other genes the child has.

In some diseases, a defect in only one gene is enough to cause the disease. In others, two defects are needed. These are called disease inheritance patterns.

A.  Carriers have a genetic defect for a disease but don't have the disease. Many people carry defective genes for autosomal recessive and X-linked recessive diseases. They're "protected" from getting the disease by their "backup" copy of a normal gene on the corresponding chromosome. But carriers can pass the defective gene to a child.

Gene defects that cause diseases only when they come in pairs become a problem when both parents give the same or a similar gene defect to their baby.

When there's a history of a recessive disease, carrier testing can be used to assist couples in planning a family.

But carrier testing can only tell a couple what the odds are of having a child with a genetic disorder. It can never predict with certainty whether a particular pregnancy will result in an affected or unaffected child, because no one can predict which of two paired chromosomes from a parent will go to a child.

Genetic (DNA) testing used to confirm a diagnosis in a child or an adult isn't the same as carrier testing. Testing to confirm a diagnosis involves looking at someone's DNA to see whether or not that person has the genetic potential to develop a disorder.

A.  The most direct and reliable way to confirm a diagnosis or test someone's carrier status for a disease is to check a particular gene and see whether the DNA has mutations known to cause that disease. This can be done when a specific disease gene has been isolated.

However, it's sometimes possible to test for a disease before a gene is found if there's a family history, if there are several affected and unaffected relatives willing to be tested, and if a region of a chromosome is known to be associated with the disease.

Doctors need samples of DNA (usually taken from white blood cells, although any cells can be used) from several members of a family, some with and some without the disease being studied. The more family members sampled, the better.

Laboratories can then examine a span of a person's DNA to see whether its size and other characteristics are like those of the affected or unaffected family members.

Though not as accurate as DNA examination using a specific gene, comparing family DNA is a fairly good way to predict whether someone will develop a disease or is a disease carrier.

A.  Duchenne muscular dystrophy is an X-linked recessive disease. Females usually don't have any symptoms, though they can carry the disease on one of their two X chromosomes and pass it to their sons.

A male child of a woman who carries the Duchenne dystrophy defect has a 50 percent chance of inheriting the gene from her normal X chromosome and a 50 percent chance of inheriting the gene from the other, dystrophy-carrying X chromosome. He'll inherit a Y chromosome from his father, so there won't be any normal X chromosome to serve as a backup. (Any sisters he has will get a normal X from their father, assuming he doesn't have Duchenne.)

So, for each pregnancy, a woman carrier of Duchenne dystrophy has a 50 percent chance of having a boy, and each boy has a 50 percent chance of having Duchenne dystrophy. Each pregnancy is a separate roll of the genetic dice, so it's possible for all the boys in a family to inherit the gene for Duchenne or for none of them to inherit it.

Even families with several children (say, five or six) aren't large enough to follow mathematical laws of probability. If you tossed a coin 100 times, about 50 of the tosses would come up heads and 50 tails, but it's possible to toss a coin five or six times and get all heads or all tails.

A.  When a child develops a genetic disease for which there's no family history, it means one of three things. It could mean that the disease is inherited in an autosomal recessive pattern, so that members of previous generations were only carriers, until this child inherited two copies of the abnormal gene.

Or, it could mean that the disease is inherited in an X-linked recessive pattern and that this is the first boy in the family. Previous generations may have had female carriers but, until a boy was born, the disease wasn't apparent.

Another explanation is that this child is the first in the family to have the genetic defect. The defect may have occurred during the development of some of the reproductive cells of either parent, or it may have occurred during the very early development of the baby in the womb. Either way, the parents didn't carry the gene in their own cells (except in some of their egg or sperm cells, if this is where the mutation occurred).

In Duchenne dystrophy, for example, about a third of children have mothers who don't carry the genetic mutation; the mutation occurs later, during the very early development of the unborn baby or of some of the mother's reproductive cells. In these situations, tests of the mother's blood cells or muscle cells don't reveal any problem.

Once a child acquires a genetic mutation, it can then be passed to the next generation.

The word "genetic" doesn't mean "inherited." It means the disease is in a person's genes and that it may have been inherited from the parents' genes. The term "inheritable" is coming into common usage to describe genetic diseases.

A.  If you aren't carriers of a disease, you have the same chance of having an affected child as any couple without a family history. It's the makeup of the parents' genes that counts in inheritance, not the genes of anyone else in the family.

A.  Screening tests are applied to large population groups sometimes those considered at high risk for a disorder, sometimes even larger groups, such as all newborn babies or all children entering kindergarten.

DNA tests are very expensive, require a great deal of technical skill and are only available for a few diseases, so screening tests don't involve looking at genes. Instead, they usually measure substances in the blood, such as proteins or sugars. Abnormal levels of these substances may signal an underlying genetic defect, which then requires more specific tests.

For example, most newborns in the United States get a blood test for phenylalanine, a substance that builds up in the blood when the enzyme that breaks it down is defective. A genetic flaw is responsible for the enzyme's malfunction and a condition called PKU (phenylketonuria).

Screening tests are only done for a handful of genetic diseases, so you can't assume that a child is free of genetic disease because he or she has had these tests.

A.  Ask your MDA clinic for a referral to a genetic counselor.