Genetics and Genetic Counseling
Genetics and Genetic Counseling
A branch of science that attempts to understand the fundamental biologic makeup of organisms by examining the genetic blueprints in each cell
The nucleus of every cell holds the key to nearly every visible and invisible feature of the human body, from the color of hair to the pumping capacity of the heart. In each nucleus of every cell there are 23 pair of chromosomes (46 total). One pair of these chromosomes determines the sex of the child while the other 22 pair determine all the other components of the human body. Chromosomes contain genes which influence the production of proteins and thus influence all aspects of body structure and function. There is a tremendous amount of information encoded in the nearly 100,000 genes in each cell Geneticists and molecular biologists work to identify the variations that exist between animals or humans by studying the changes that occur during the cell's division. The alterations that take place during the development of any organism may include mutations, insertions, deletions, or translocation during the copying of genetic material from one cell to the other. These changes are the basis for chromosomal abnormalities such as in Down syndrome or trisomy 18, where there is an extra or missing chromosome material in the embryo, or in singlegene disorders like sickle-cell anemia and cystic fibrosis, which are caused by a small change on a single gene called a point mutation.
The study of human genetics is less than 100 years old and yet in the last century scientists have identified over 400 genes that cause a variety of diseases from sickle-cell anemia and Down syndrome to high cholesterol and depression. In addition science has been able to elucidate the inheritance pattern of disease in certain families.
A genetic counselor works with a person concerned about the risk of an inherited disease. In 1975, the American Society of Human Genetics clarified the role of genetic counseling. As a communication process, genetic counseling attempts to 1) accurately diagnose a disorder, 2) access risk of recurrence in the concerned family members and their relatives, 3) provide alternatives for decision-making, and 4) provide support groups that will help the family members cope with the recurrence of a disorder.
The role of the genetic counselor is to facilitate the exchange of information regarding a person's genetic legacy. The genetic counselor does not prevent the incidence of a disease in a family but can help family members assess the risk for certain hereditary diseases and offer guidance. At present there are less than 2,000 accredited
GENES AND BEHAVIOR
Is a child's athletic ability inherited, or simply a product of training? If one parent has schizophrenia, will his child acquire the disease? The genetic foundations of behavior are studied by behavior genetics, an interdisciplinary science which draws on the resources of several scientific disciplines, including genetics, physiology, and psychology. Because of the nature of heredity, behavior geneticists are unable to assess the role played by genetic factors in an individual's behavior: their estimates by definition apply to groups. There are 23 pairs of chromosomes in each human cell (a total of 46 chromosomes—each with approximately 20,000 genes). Genes from both members of a pair act in concert to produce a particular trait. What makes heredity complex and extremely difficult to measure is the fact that human sperm and eggs, which are produced by cell division, have 23 unpaired chromosomes. This means that one half of a person's genes comes from the mother, and the other half from the father, and that each individual, with the exception of his or her identical twin, has a unique genetic profile.
Scientists are currently working on the Human Genome Project, which will map the estimated 100,000 genes in the human DNA. So far, they have been able to identify genes responsible for a variety of diseases, including Huntington's disease, Down syndrome, cystic fibrosis, Tay-Sachs disease, and a number of cancers. Genetic information about a particular disease constitutes a crucial milestone in the search for a cure. For example, phenylketonuria (PKU) is a disease caused by a re cessive gene from each parent; PKU's genetic basis is clearly understood. A child with PKU is unable to metabolize phenylalanine, an amino acid found in proteins. The phenylalanine build-up afflicts the central nervous system, causing severe brain damage. Because the genetic processes underlying PKU are known, scientists have been able to develop a screening test, and thus can quickly diagnose the afflicted children shortly after birth. When diagnosed early, PKU can be successfully controlled by diet.
While genetic research can determine the heritability of a some diseases, the genetic foundations of behavior are much more difficult to identify. From a genetic point of view, physical traits, such as the color of a person's hair, have a much higher heritability than behavior. In fact, behavior genetics assumes that the genetic bases of an individual's behavior simply cannot be determined. Consequently, researchers have focused their efforts on the behavior of groups, particularly families. However, even controlled studies of families have failed to establish conclusive links between genetics and behavior, or between genetics and particular psychological traits and aptitudes. In theory, these links probably exist; in practice, however, researchers have been unable to isolate traits that are unmodified by environmental factors. For example, musical aptitude seems to recur in certain families. While it is tempting to assume that this aptitude is an inherited genetic trait, it would be a mistake to ignore the environment. What is colloquially known as "talent" is probably a combination of genetic and other, highly variable, factors.
More reliable information about genetics and behavior can be gleaned from twin studies. When compared to fraternal (dizygotic) twins, identical (monozygotic) twins display remarkable behavioral similarities. (Unlike fraternal twins, who develop from two separate eggs, identical twins originate from a single divided fertilized egg.) However, even studies of identical twins reared in different families are inconclusive, because, as scientists have discovered, in many cases, the different environments often turn out to be quite comparable, thus invalidating the hypothesis that the twins' behavioral similarities are entirely genetically determined. Conversely, studies of identical twins raised in the same environment have shown that identical twins can develop markedly different personalities. Thus, while certain types of behavior can be traced to certain genetic characteristics, there is no genetic blueprint for an individual's personality.
Twin studies have also attempted to elucidate the genetic basis of intelligence, which, according to many psychologists, is not one trait, but a cluster of distinct traits. Generally, these studies indicate that identical twins reared in different families show a high correlation in IQ scores. No one questions the genetic basis of intelligence, but scientists still do not know how intelligence is inherited and what specific aspects of intelligence can be linked to genetic factors.
genetic counselors practicing in the United States. This figure is expected to increase in response to the enormous changes taking place both in the scientific community and society. There are limitations to the power of genetic counseling, though, since many of the diseases that have been mapped offer no cure for such disorders as Down syndrome or Huntington's disease. Although a genetic counselor cannot predict the future unequivocally, he or she can discuss the occurrence of a disease in terms of probability. A genetic counselor, with the aid of the patient or family, creates a detailed family pedigree that includes the incidence of disease in first-degree (parents and siblings) and second-degree relatives (aunts and uncles). Before or after this pedigree is completed, certain genetic tests are performed using DNA analysis, χ ray, ultrasound, urine analysis, skin biopsy, and physical evaluation. For a pregnant woman, prenatal diagnosis can be made through amniocentesis (the withdrawal of amniotic fluid during pregnancy) or chorionic villus sampling (the biopsy of chorionic villus tissue).
Concerns about genetics research is an issue that will become increasingly relevant to families trying to embark on uncovering their genetic risks towards common diseases such as breast cancer, heart disease, asthma, depression, and diabetes. Even as each month brings a new gene discovery, barriers still remain between the concerned person and the wealth of information that lies in one's DNA. Access to genetic centers and counseling may be further hampered by insurance companies that do not reimburse patients for testing and counseling.
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