Table of Contents | Chapter 3
Priya Should Find Out She
We will have to learn this language if we want to learn all the secrets of the genes. So this is exactly what scientists from around the world have decided to do. They have set out to learn the language of the genes. This international effort is called the Human Genome Project. The United States is spending 3 billion dollars over 15 years on this project. Other countries also are investing large amounts on research. It is a huge undertaking that involves researchers in biology, physics, engineering, computer science, and many other fields.
The task they have taken on is challenging and exciting, but difficult. Genes are made of a threadlike material called DNA, which itself contains chemical ingredients called bases. There are only four bases, but they repeat one after the other in an ever-changing order throughout the genes. Think of the four bases as letters of the alphabet, combining together in some strange language to spell out each gene's instruction.
|This close-up shows the four bases that make up the genetic code. The bases are thymine (T), adenine (A), guanine (G), and cytosine (C).|
With just four letters, the alphabet of this mysterious language is very short. However, the words written in this language are not short at all. A single gene has thousands of bases. Some genes have millions of bases. So each gene is like a single word with thousands or millions of letters to it. In addition, it is hard to figure out where each gene begins and ends, because the spaces in between also are filled with long strings of letters.
The immediate goal of the Human Genome Project is to put together a copy of the human instruction book, letter by letter. Having this copy to read will make it easier to decipher the language of the genes. But making the copy is very difficult. First, scientists have to get inside a cell. Then they have to get inside the nucleus of the cell to the DNA. The DNA is curled into tight coils, so they have to uncurl it. Then they have to look at the DNA to see which of the four bases comes first, which second, which third, and so on. Then they have to write this down.
It may sound simple, but it isn't. This is a job that involves unthinkably small objects and incredibly large numbers. It would seem to be an impossible task. Yet, because of advances in computers, microscopes, chemical analysis, and other tools of science, it is a job that is only a few years from being done.
Once the Human Genome Project has made its copy of the human instruction book, there will still be the task of translating what it means.
And even when this translation has been done, it will only be a model. It will not reveal exactly what is written in your genome or in any other particular person's genome. This is because every human being is different. Each person's genome is unique.
Your unique genome was given to you by your parents. This took place in the process of conception. Conception is when a man's sperm fertilizes a woman's egg.
Sperm and eggs are made from special cells called germ cells. (The word "germ" is from the Latin word for "seed" or "bud".) Germ cells are found in the male's testes and in the female's ovaries. Like most other cells, each germ cell has a copy of the genome inside it. The genome is not one long strand of DNA. Rather, it is divided into separate strands called chromosomes, each containing several thousand genes. Human beings have 46 chromosomes.
|Conception happens when a man's sperm enters a woman's egg and fertilizes it.|
To make sperm (in a man) or eggs (in a woman), a germ cell goes through a complicated process. First, it makes a copy of each chromosome. Then it divides, twice, to form four sperm or four eggs. Its double set of chromosomes is sorted equally among the four sperm or eggs, so that each has a half set of 23.
When a sperm and an egg join together, they combine their half sets to make a unique, new set of 46 chromosomes. There are so many different ways that your parents' chromosomes can be combined, that the chance of the same mix happening twice is close to zero.
The chance becomes truly zero because of something that happens as sperm and egg are formed. That something is called crossing over. It happens when a germ cell is making copies of its chromosomes before dividing. In crossing over, a section of one chromosome switches places with the same section from its pair. This makes the copied chromosome a little different from the original. It will carry a slightly different mix of genes.
The chromosomes you inherit from each parent correspond in size and in the genes they carry. The one exception to this rule involves the two chromosomes that determine sex, nicknamed X and Y. A person who inherits two X chromosomes (XX) is female, while a person who inherits one X and one Y chromosome (XY) is male. The X chromosome is larger and contains genes that are not found on its partner Y chromosome.
The fact that you have pairs of chromosomes means that you have pairs of genes for every trait (except for those traits found only on the X chromosome). So which instruction in each pair gets followed? It depends on whether the genes in the pair are dominant or recessive. In any pair, if one gene is dominant over the other, its instructions are followed. A recessive gene's instructions come into play only if neither gene in its pair is dominant.
For example, the gene for Type A blood is dominant over the gene for Type O blood. Therefore, if you inherit the Type A gene from one parent and the Type O gene from your other parent, you will have Type A blood. The gene for Type B also is dominant over the gene for Type O. So if you inherit one Type B gene and one Type O gene, you will have Type B blood. Because the Type O gene is recessive, you will have Type O blood only if you inherit two Type O genes, one from each parent.
Something very interesting happens, however, if you inherit one Type A gene and one Type B gene. In this case, you will have Type AB blood. The instructions of both genes come into play because neither dominates over the other. In fact, many of your traits are shaped by both genes in a pair. In addition, most traits are affected by more than one gene. For example, the shade of your hair is affected by many pairs of genes working together.
When a gene becomes active, it leads to the production of a protein. Proteins are the basic chemicals that make up the structure of cells and direct their activities. The human body produces thousands of different proteins. Most every protein has a different function, although there is some overlapping of jobs.
For example, one protein carries oxygen in the blood. Another protein regulates the salt in your sweat. For every function of your body, proteins are involved, and the production of these proteins is regulated by genes.
A gene in one person may carry a slightly different instruction than the corresponding gene in another person. Most variations in the same gene don't cause any problems to health. For example, a person whose genes lead to blue eyes can see just as well as a person whose genes lead to brown eyes.
Variations in the instruction carried by a gene come about through a process called mutation. A mutation is a change that occurs to the order of the bases appearing in the DNA inside a cell. Mutations can happen to any gene inside any cell of your body at any point in your life. However, most mutations occur as germ cells make copies of their chromosomes before dividing to form sperm or eggs. In this process, millions of bases must be copied in exactly the right order. Mostly, they are. However, with every copy of DNA, there are some errors. A base is put in the wrong place or is left out. Sometimes extra copies are made of a string of bases or of whole chromosomes.
|Inherited mutations are carried in the DNA of reproductive cells. When reproductive cells containing mutations combine to produce offspring, the mutation will be present in all body cells of the offspring.|
These kind of changes can alter the order of bases in the affected genes. Therefore, the instructions that direct the production of proteins may change. This can affect the traits that show up.
Many mutations happened to the genes of people who lived long ago, and these mutations have been passed down through the generations. Others are new, occurring to a person during his or her lifetime. New mutations may be passed on to the next generation if they appear in sperm or egg cells.
Most mutations are harmless because they don't result in any important changes to traits. Some mutations are important because the new trait is helpful to the survival of a species. However, sometimes mutations cause problems in how your body functions. These problems are called disorders.
An interesting example of a mutation is the one that causes red blood cells to take on an unusual "sickle" shape, like the curving blade of a knife called a sickle that is used to cut tall grasses. This gene variation is found mainly in people who live near the equator (or whose ancestors did). Researchers believe that inheriting one gene with this particular variation may be helpful against malaria, which is a disease caused by parasites transmitted by mosquitoes into the bloodstream. The parasites feed off red blood cells. However, they have a hard time feeding off sickle-shaped cells. This limits the damage they can do inside the body.
People who inherit two copies of this gene variation, however, are affected by a disorder called sickle cell anemia. They have so many sickle-shaped red blood cells that sometimes it is hard for the blood to flow. The symptoms for sickle cell anemia range from mild to severe. Although treatment has improved in the past few decades, many people still become very sick and die from the disease.
In the past few years, researchers have learned how to test for hundreds of genetic disorders. More new tests are coming out all the time. These tests can be very helpful for diagnosing disorders in children and adults. The tests also can be used to predict the chances that a person will come down with a particular disease later in life.
Genetic tests are sometimes used by couples who want to learn their risk of passing on genetic disorders to any children they might have. Other genetic tests are prenatal. That is, the tests are done before birth to an embryo or fetus, to see whether it has any genetic problems.
There are many different kinds of tests. The test used depends on what disorder is being looked for and what is known about the gene, such as its location in the genome or the protein it controls. In some cases, the test results are definite. In other cases, the test results only suggest what the person's risk is for developing a disorder.
One way of testing is to do a medical exam. Doctors examine the person to see if he or she shows signs of the disorder. They also may study the person's family history for clues as to how the disorder has been passed down from generation to generation.
To find some disorders, doctors may take a blood sample in order to look at the person's chromosomes under a microscope. They may test the blood sample to find proteins that would reveal a gene at work. Very advanced tests can look inside a section of a chromosome to "read" the DNA.
Great progress has been made in genetic testing in recent years. However, progress in the treatment of genetic disorders has been much slower. That is the problem for Priya, the woman who fears that she may have inherited Huntington's disease (HD).
HD affects 1 in every 10,000 people. A mutation in one gene means that the protein it instructs the body to make gets produced abnormally. For reasons that are not yet clear, this leads to a breakdown in the parts of the brain that control movement.
The HD gene is dominant. This means that only one gene in a corresponding pair needs to carry the HD mutation in order for the disease to take hold. Priya's mother had the gene and, therefore, the disease. If Priya inherited the HD gene from her mother, she too will someday develop the disease. This will be true even if the corresponding gene she inherited from her father does not have the HD mutation.
|A doctor looks at the results of a genetic test.|
Genetic tests are available to tell Priya if she carries the gene for the disorder. However, no treatment is available. If Priya finds out through testing that she has the gene, there is little she can do. No one knows how to ward off the disease or keep it from getting worse. No one can even tell when the disease will hit or how quickly it will take over her body.
In other words, the only real use of the test is to tell Priya how she stands, one way or the other. Such news could change her life, but not necessarily in the way she expects.
For example, you might think that Priya would be happy to find out from testing that she has been spared the disease. However, it is possible that she will feel "survivor's guilt" if other members of her family have not been spared. It also is possible that once Priya stops worrying about HD, she may discover other problems in her life that she has been avoiding. Finding out that she doesn't have HD may be happy news, but it will not necessarily make Priya a happy person.
You might also think that Priya would be sad to find out from testing that she will get the disease. But Priya may discover that it is a big relief to know for sure what she has always feared. It may give her a new appreciation for each day that she has. She may feel that she can now make better decisions for the life she has left.
The fact is, Priya cannot know ahead of time how she will react to the test results. And she cannot predict other problems that may come as a result of testing. For example, in order to find out how HD is passed down in her family, doctors may want to test her close relatives, such as her brothers, sisters, aunts, and uncles. This is called a genetic linkage study. Priya will have to ask them to take part in the study. Asking such a thing can be very difficult. It may put extra pressure on her when she already has a lot on her mind. The relatives may feel pressured themselves. Or they may feel guilty if they refuse to take part.
Here's another problem Priya must consider: Suppose the test result says that she has the HD gene. Who else should know? Does her boyfriend have a right to know before they make plans to marry and have children? When does she tell her employer, now or when she becomes ill? What about her health insurance company, which at some point may have to provide expensive medical care for Priya?
Priya must decide how to handle the information in a way that is fair to herself and to others. However, it may not be entirely in her control. She may want to keep it secret, but have a hard time doing so. Or a relative who took part in the testing may reveal information to others. If information on the results of the test is put into her medical records, both her employer and her health insurance company may find out without her telling them. This could cause her to lose her job, which would be illegal, but still could happen. Or it could cause her to lose her health insurance, which has indeed happened to others in this situation.
Another thing that Priya must think about is that testing could take a lot of time and be expensive. She could have a long, distressing wait for results. The answer may not be clear cut. It is even possible that the answer will be wrong because of a lab error or because the test itself is not perfect.
Priya's decision whether to get tested may be affected by recent news. In 1993, researchers located the gene in which the HD mutation occurs. This discovery means that scientists can develop a simpler and more reliable way to test for HD. The information also is useful for scientists working on a cure. Treatment may come in time to help someone like Priya. Then again, it may not.
|Genetic counselors are trained to help people make decisions about their health.|
HD is unusual because if you carry the gene for the disorder, you get the disease for sure and it kills for sure. Most genetic disorders are not so direct. But the issues surrounding testing remain.
That's why many people feel that genetic counseling is extremely important for anyone wanting to get information about their genes. With counseling from professional advisors who are experts on genetics, people like Priya can understand the facts of their situation. Counselors can help them clearly understand the limitations of tests and think through ahead of time how different test results might affect them. Finally, counselors can explain to people what their choices are once they know the results.
However, genetic counseling is a new field, and there aren't very many trained genetic counselors. Genetic testing is fast becoming a billion-dollar industry. New tests are coming out faster than new counselors are being trained. There simply aren't enough genetic counselors for everyone who should have counseling.
This problem is probably going to get worse before it gets better. Companies that sell tests want to make a profit, so they will be marketing them not just to genetics specialists, but to all doctors and directly to the public. As people hear about new tests, demand for them will surely increase. This is especially likely to happen as the tests become cheaper, more accurate, and easier to perform. People getting tested may not realize that they need to have the results explained to them. They may not know how to ask for this. Also, doctors who perform the genetic tests, but who do not have special training in ,may not be able to answer their patients' questions.
Some people who have genetic tests may not get counseling even if they want it, because their insurance company will pay for testing, but not for the counseling to go with it. All of this means that a lot of people may be getting very serious information from tests without getting the support they need to understand the results and to make good decisions.
There's one more reason why genetic counseling is so important. With many genetic disorders, genes are only one of the factors involved. Other factors, such as lifestyle, play a part. Counselors can help people understand what they can do to avoid triggering a genetic disorder. These kinds of choices are covered in our next chapter.
|Table of Contents | Chapter 3|
Your Genes, Your Choices is a publication of Science + Literacy for Health, a project of the AAAS Directorate for Education and Human Resources. The publication was funded by the U.S. Department of Energy. The website was built by Mike Wooldridge. Send feedback to SciLit@aaas.org.