Table of Contents | Chapter 5
Carlos and Mollie Can Have a
Another common prenatal test is ultrasound imaging. Ultrasound uses sound waves to create an image of the baby inside the mother. Many physical problems can be detected with ultrasound. If something does not look right, the doctor may recommend more tests.
Amniocentesis (called "amnio" for short) and chorionic villus sampling (CVS) are tests that check for defects in the chromosomes. Doctors remove some cells surrounding the fetus. These cells are treated with a special dye and photographed through a microscope. In the photograph, the chromosomes will look striped, because of the dye. Corresponding chromosomes will have the same pattern of stripes and be the same size and shape. (There is one exception: the pair of sex chromosomes that men inherit, X and Y, will not match.) A special technique is used to rearrange the chromosomes into pairs. The picture that is created is called a karyotype. The karyotype makes it easy to see if any chromosomes are missing or broken, or if there are any extra chromosomes.
|DNA markers can be seen as black bands when properly stained and viewed under an electron microscope.|
Amnio and CVS are also used to obtain cells for analysis at the DNA level. If there is a risk that the fetus will inherit a particular genetic disorder, the gene involved can be looked at to see if it is a disease-causing variation. If the location of the gene is not known, the DNA can be searched for the presence of a DNA marker. This is a gene or other fragment of DNA whose location is known and that often is inherited along with the disorder.
With amnio and CVS, the procedure for obtaining cells is uncomfortable for the woman. Plus, there is a slight risk of miscarriage. Researchers are now developing a test that they hope will be totally safe and easy. For this test, all that will be needed is a sample of the mother's blood. The new test will examine cells from the fetus that have become mixed in with the mother's blood. Because this test will be safe and simple to perform, it could become a standard part of health care for all pregnant women.
Advances in prenatal testing have come hand in hand with advances in medically assisted ways of making babies. The term for this kind of help is reproductive technology. Reproductive technology is often used to help people who have problems conceiving children. It also is used to help people reduce their risk of having babies with genetic disorders.
|Medical technology has given new hope to childless couples.|
One form of reproductive technology is artificial insemination. This is where a sample of sperm is taken from a man, cleaned in a special way, and then injected into his partner's uterus. If the male in a couple carries a mutated gene causing a dominant disorder, the couple may choose artificial insemination using sperm given by a donor. The couple might also make this choice if they both carry the same mutated gene for a recessive disorder. In such a case, they would use a donor who does not carry a problem version of the gene. This would protect the child from inheriting the recessive disorder.
Another form of assisted baby-making is called in vitro fertilization. A common term for this has been "test tube babies." (In vitro means "in glass.") Actually, no test tubes are used. What happens is that a minor operation is performed on a woman to remove some of her eggs. These are mixed with sperm from her partner. Eggs that become fertilized are then implanted in the woman's uterus, with the hope that one or more will grow into a fetus. In vitro fertilization with donated eggs is sometimes used when the female of a couple carries a gene mutation that causes a dominant disorder or when she carries a gene mutation for a recessive disorder that her partner also carries.
Ex utero genetic testing is yet another new technology used in reproduction. (Ex utero means "outside the uterus.") The testing is done after eggs and sperm have been mixed together using in vitro fertilization. For any eggs that become fertilized, cells are removed for DNA analysis to see if gene variations that lead to disorders are present. Embryos that do not carry problem versions of the genes are then implanted in the mother's uterus.
Using reproductive technology may not be a very romantic way to have children. However, combined with prenatal testing, reproductive technology has spared thousands of couples the tragedy of giving birth to a baby with a terrible genetic disorder.
Prenatal testing and reproductive technology are giving more people the opportunity to be parents of healthy children. However, along with these opportunities come decisions that must be made. This is what is creating a problem for Mollie and Carlos, the couple that want to have a baby.
Carlos doesn't want the baby to inherit CF. CF is a recessive disorder that affects 1 in every 2,500 white babies. (It is much less common in babies of other races.)
Scientists have discovered the gene involved in CF. Many different mutations of this gene lead to CF. A test is available that can identify whether a person has one of the more common mutations linked to the disease. Carlos has taken this test and has been found to carry a CF mutation. Since CF is a recessive disorder, he risks having a child with CF only if Mollie also has a CF mutation. If that is the case, their chance of having a child with the disease is one in four. The risk is the same each time they have a child.
Whether Mollie should get tested or not is just the first decision on a road of choices for this couple. If Mollie refuses, perhaps Carlos will back down from his position, and they will take their chances with a child. Or perhaps Carlos will look for another mate who will agree to take the test. Or perhaps Mollie will look for another mate who is not so afraid of the risk. Then again, perhaps the couple will decide to stay together and not have children, or they may choose to adopt children instead.
If Mollie agrees to be tested, there are two possible results. The test will be positive if she has one of the more common mutations linked to CF. It will be negative if she is not a carrier or if she is a carrier for an unusual mutation leading to CF. This means that even with a negative test result, there is a small chance that Mollie and Carlos could still have a child with CF.
If Mollie tests positive, she and Carlos move on to the next choice -- whether to have a child together. Here again they may want to consider finding new partners, not having children, or adopting. If they decide to go ahead and conceive together, their next choice is whether to have prenatal testing done on the fetus to see if both of its genes carry a CF mutation. If they have the prenatal test done and it is positive, they face the decision of keeping or aborting the pregnancy.
Their decision will be affected by their beliefs about abortion. It also may be affected by new findings about CF. Modern treatments can keep many people with CF alive into their thirties. Another important new finding is that some people who inherit two CF-mutated genes do not get the full disease. Some only suffer from asthma or lung infections. Some males cannot father children, but have no other health problems. Right now, the genetic test for CF can't always tell how severe the disease will be. If Mollie and Carlos learn through prenatal testing that their fetus has two genes with CF mutations, they still won't know how bad the news is. The baby that is born may become extremely sick or may stay fairly healthy.
Because of this uncertainty, the couple may decide to continue the pregnancy of a fetus that tests say has two genes with CF mutations. In that case, once the child is born, they can have him or her regularly checked for symptoms so that treatment can begin as early as needed. And they will want to keep up on the latest developments in the treatment of CF, because medical research may well lead to better treatments in time to help their child.
Based on his own family experience, however, Carlos may be determined not to have a child with CF. In this case, the couple has other choices. One possibility is to use an egg or sperm donor who does not carry a CF mutation. Another possibility is ex utero genetic testing. Mollie's eggs and Carlos's sperm would be mixed together. Cells from any eggs that become fertilized would be removed for DNA analysis. The embryos that do not carry genes with the CF mutation could then be implanted in Mollie's uterus. The child would be theirs, and he or she would not develop CF.
There are many other factors that complicate the decisions Mollie and Carlos must make. Suppose Mollie takes the test and it is positive. Suppose also that she and Carlos decide to go ahead and have a baby through in vitro fertilization, using either a donor who does not have the CF form of the gene or their own sperm and egg along with ex utero genetic testing. They will have to face the fact that this procedure can cost tens of thousands of dollars and may not be covered by their health insurance. In vitro fertilization also has a high failure rate and may take many tries before it works. This means a lot of time-consuming visits to medical offices. Mollie and Carlos will have to decide how long to keep trying if in vitro fertilization doesn't succeed right away.
If the couple decides to go with an egg or sperm donor, they will have to decide which of them gets to contribute their genes to their future child and which does not. They also will have to select a donor -- either a friend, a relative, or a person unknown to them. They will have to decide what kind of relationship to have with their donor.
Mollie and Carlos also have to realize that even if they do everything in their power to avoid having a child with CF, they could still end up with a sick baby. After all, CF is only one of many possible genetic disorders. It would be far too costly and time consuming to test for all of them. Besides, there is no test for many genetic disorders at the present time. It's also important to remember that many health problems do not have genetic causes. For example, a difficult delivery could cause medical complications in the child. Whatever Mollie and Carlos do to add a child to their family, they have no guarantee that he or she will have perfect health.
In the past, there were few options for couples like Mollie and Carlos. Medical advances have opened up an overwhelming number of choices. The same is true for couples that have disorders other than CF in their family trees. As more genes involved with disorders are located, more screening tests will reach the market. Their use may be encouraged by the companies that make them, by many people in medicine, and by popular demand. Soon, all couples planning to have babies could face decisions like those that are before Mollie and Carlos.
What if you could decide ahead of time the features of your baby, such as the hair color and body shape? Scientists believe that prenatal testing may someday be used not only to avoid disease, but to select for desired features.
In one way, this already is being done. Some parents are using prenatal tests to choose the sex of their child. In some countries in which the culture values boys more than girls, ultrasound and amnio are used mainly to check the sex of the fetus. If the fetus is a girl, it is aborted. In the United States, sex selection is condemned by most people. However, some parents do it anyway.
At the present time, genetic testing does not reveal much else about a child's features. There could come a time, however, when genetic testing makes available endless choices. Parents will then have many more decisions to make -- starting with the decision as to whether it is right to make such choices.
Genetic testing has another important use besides screening for disorders and traits. It also can be used for personal identification. You'll read about this in our next chapter.
|Table of Contents | Chapter 5|
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.