Genetic Screening

Genetic Screening

Today, genetic screening and counseling help parents to become more educated and to make choices about having children. Newly born infants are reg-ularly screened for anatomical disorders such as ­congenital hip dysplasia and imperforate anus. Anatomical defects are usually surgically corrected. Infants are also tested for metabolic diseases such as PKU. This condition is managed by strict diets that exclude most foods containing phenylalanine. Genetic screening alerts parents of needed treat-ments in order to keep their children healthy. Adult children of parents who have Huntington’s disease are also helped by genetic screening and counsel-ing. Women who become pregnant at age 35 or later may seek genetic screening for the possibility of their babies having trisomy 21, also known as Down syndrome. This condition occurs more often in the babies of older mothers. Genetic screening can occur before a baby is conceived by recognizing carriers of genetic conditions or during fetal testing.

Carrier Recognition

Carriers of genetic conditions are identified in two basic methods:

■■ Pedigree: The tracing of a genetic trait through several generations, which helps to predict future outcomes; phenotype information on as many fam-ily members as possible is used; a pedigree is also called a “family tree”; rules of dominant-recessive inheritance are applied to deduce genotypes of par-ents and determine genotypes of others in the same generation as the parents (FIGURE 27-5)

■■ Blood tests: Used to screen for sickling genes in heterozygotes; blood chemistry tests and DNA probes detect presence of other unexpressed, recessive genes such as carriers of Tay-Sachs dis-ease and cystic fibrosis.

Fetal Testing

When there is a known risk of a genetic disorder, fetal testing is performed. The most common method is via amniocentesis. A wide bore needle is inserted through the mother’s abdominal wall into the amni-otic sac. Approximately, 10 mL of fluid is withdrawn. This procedure is usually performed after the 14th week of pregnancy, so that the fetus is not injured by removing the needed amount of fluid. Ultrasound is used to visualize the fetal position as well as the posi-tion of the amniotic sac. This procedure is extremely safe. The amniotic fluid is checked for enzymes and other chemicals that are markers for certain diseases. Most tests are performed on the sloughed-off fetal cells found in the fluid. For over several weeks, the cells are cultured in laboratory dishes, then examined for DNA markers of genetic disease. They are karyo-typed to check for chromosomal abnormalities.

Another procedure called chorionic villus sampling (CVS) uses suctioning of parts of the chori-onic villi from the placenta through a catheter inserted into the cervix. One of the main advantages of having this test instead of amniocentesis is that it can be done earlier in pregnancy. Ultrasound is used to guide this insertion to a region where a piece of placental tissue may be removed. This procedure can be performed as early as the eighth week of pregnancy, though the 10th week is recommended. Karyotyping can be performed almost immediately. The rapidly dividing chorionic cells can, therefore, be assessed at a much earlier time than is possible with amniocentesis. With CVS, there is an increased risk of defects of the fingers and toes of the fetus. Along with amniocentesis, CVS is regularly ordered for pregnant women over 35 due to the higher risk of Down syndrome. Both procedures are also per-formed on younger mothers when the likelihood of a severe fetal disorder is higher than the probability of any harm caused by the procedures.

Both amniocentesis and CVS will become obso-lete because of advances in the sequencing of DNA. Today, maternal blood samples can be obtained easily, to examine free-floating maternal DNA as well as the DNA of the fetus that is released from the placenta. Maternal blood samples are tested for Down syn-drome and other fetal chromosomal abnormalities. Similar blood testing can give information about the fetal genome, identifying mutations that may make it more likelihood for certain diseases to develop.

Human Gene Therapy

Human gene therapy is helping to treat or cure vari-ous disorders, especially those caused by a single gene or protein that is defective. It is possible to insert a virus that contains a functional gene into defective cells. Another method is to inject DNA that has been “corrected” into a patient’s cells. These therapies have shown varying results in treating muscular dystrophy and cystic fibrosis. Genetic engineering is still very expensive, however, and there are opposing opinions about its ethics, infringement on religious beliefs, and how it can or should be used.

1. What common disorders are routinely checked for by using genetic screening?

2. Describe a “pedigree.”

3. Contrast how amniocentesis and CVS are performed.