Genetic screening is one of the most controversial topics in the scientific arena today. The advent of the Human Genome Project, which maps the complete human genetic code, has brought this issue to the forefront. This paper will discuss the basic science that underlies genetic screening, applications of genetic screening, and investigate some of the common misconceptions and ethical questions about its use.
Genetic screening itself is simply "the systematic search within a population for persons possessing particular genotypes, which are either associated with disease, predisposing to disease, or leading to disease in descendants" (Miller). In simpler terms, genetic screening involves testing and determining whether "an individual's genetic material to predict present or future disability or disease either for oneself or one's offspring" (McCarrick). Essentially, genetic screening is conducted for several basic reasons, including the care of the ill and the prevention of disease, providing reproductive information, determining the incidence of disorders in the general population, and research (Miller).
In recent years, the incidence of genetic screening has increased rapidly. Tests for cystic fibrosis jumped from 9310 tests in 1991 to 63,000 tests in 1992, according to the U.S. Congress' Office of Technology assessment. The National Institute of Health has since recommended routine cystic fibrosis testing for all six million American women who become pregnant each year (McCarrick).
The potential for the growth of the application of genetic screening is tremendous. As genes that are linked to breast cancer are identified, the idea of regular genetic testing for all women is becoming more realistic. Further, as the population ages, interest in genetic testing for age-related diseases like Alzheimer's disease continue to grow (McCarrick).
Scientific Basis of Genetic Screening
An understanding of genetic screening rests solidly on a basic understanding of genes and DNA. DNA (short for deoxyribonucleic acid) is simply a nucleic acid inside a cell's nucleus that contains the basic genetic instructions for the biological development of the organism. DNA is made of a famous double helix structure that resembles a spiral staircase. Each of the "rails" or sides of the DNA double helix is made of a stand of DNA, which is made up of a sugar, a phosphate, and one of four DNA base nucleotides (adenine (A), thymine (T), cytosine -, and guanine (G)). Each base can only hydrogen bond to another (A binds to T, and C. binds to G), therefore the identify of base pairs on one strand determines the identity of bases on the opposing strand. When the DNA double helix is separated, each strand can act as a template to replicate the other side (Alberts).
The sequence of nucleotides (bases) on a DNA strand is crucial. The sequence of nucleotide identifies a specific protein, which is the basic building block of the organism. Each series of three nucleotides (a codon) codes for a specific amino acid. Specific combinations of amino acids make up a particular protein. The relationship between the amino acid sequence of the protein and the nucleotides is known as the genetic code.
Genes are pieces of DNA that are known as the "functional and physical unit of heredity passed from a parent to offspring" (Genetic Science Learning Center). Most genes contain the information for making a specific protein. In molecular biology, genes are segments of DNA within chromosomes (Alberts).
A chromosome is simply a long, continuous piece of DNA. Humans have 46 chromosomes. Within humans, somatic cells (cells of the body) are diploid, meaning there are two sets of chromosomes, one from the mother, and one from the father. In contrast, the gametes (the eggs and sperm) are haploid, meaning they contain only one set of chromosomes. When an egg is fertilized by a sperm, the single set of chromosomes from each parent join to make a cell with a double set of chromosomes. There are many types of chromosomal aberrations that lead to disease within humans. Likely the most common of these disorders is seen in Down's Syndrome, where individuals have an extra chromosome 21, leading to mental retardation. In Turner syndrome, an individual has one X chromosome, rather than an XX (female) or XY (male). This leads to the development of underdeveloped female sexual characteristics (Alberts).
A mutation is a permanent and transmissible change to the genetic material of an organism. Mutations can occur within cell division or due to exposure to chemicals, death or malfunctioning of the cell. Rarely, mutations can be favorable, and these are thought of as a main driving force in the theory of natural selection. In contrast, neutral mutations do not affect the organism and can build up over time. Many mutations do not cause disorders because they are repaired. Given that humans have two copies of each chromosome, and two copies of each gene, the redundant gene can usually "take over" for the mutant gene (Genetic Science Learning Center; Alberts).
Mutations can be naturally occurring (spontaneous) or caused by mutagens like chemicals or radiation (induced). There are three main types of mutations: point mutations, insertions, and deletions. Point mutations occur when a single nucleotide is exchanged for another, and are often caused by a problem during replication or chemicals. Insertions occur when one or more extra nucleotides are inserted into the DMA, and can cause a shift in the reading frame of DNA, which can profoundly impact the protein created from the DNA. Deletions occur when one or more nucleotides are removed from the DNA, and can result in an alteration of the reading frame of the DNA. These are irreversible (Alberts).
Genetic screening is simply the search for specific genetic disorders. These genetic disorders are caused my mutations in a specific gene or sets of genes. These mutations result from changes to the DNA sequence of a specific gene. Mutations can occur any time from fertilization until death (Genetic Science Learning Center).
Genetic disorders can be separated into four main categories. These are chromosome abnormalities, single-gene disorders, multifactorial disorders and mitochondrial disorders (Genetic Science Learning Center).
Chromosome abnormalities occur when entire chromosomes or large segments of human chromosomes are altered, duplicated, or missing. Turner's syndrome and Down's syndrome are examples of chromosome abnormalities. In individuals with chromosome abnormalities, the karotype (a display of chromosomes within a single cell) is altered. Other examples of genetic disorders that result from chromosomal abnormalities are: Klinefelter Syndrome, Cri du chat Syndrome, Williams Syndrome, Reciprocal Translocation: Philadelphia Chromosome, and Robertsonian Translocation (Genetic Science Learning Center).
Single-gene disorders occur when a mutation results in the alteration or disappearance of a protein from a single gene. Sickle cell anemia is an example of a single-gene disorder (Genetic Science Learning Center).
Multifactoral disorders occur when multiple genes are mutated, and are often associated with environmental causes. These disorders are difficult to treat and study, and include diabetes, cancer, and heart disorders (Genetic Science Learning Center).
Mitochondrial disorders are rare, and caused by the mutations of non-chromosomal DNA within the mitochondria. Mitochondria are often called the cell's powerhouse, and are organelles that use sugar and oxygen to make energy. There are many mitochondria within a single cell, and each mitochondria contains its own DNA. Mitochondrial DNA comes from the mother (Genetic Science Learning Center).
Application of Genetic Testing
There are five main areas of focus within genetic testing. These are: prenatal diagnosis, newborn screening, carrier screening, forensic screening, and susceptibility screening (McCarrick).
In prenatal diagnosis, a fetus is identified as to whether it is at risk for a number of genetic traits or diseases. Amniotic fluid, fetal cells, and maternal or fetal blood cells are obtained in alpha fetoprotein assays of chorionic villus sampling, amniocentesis testing, or, less invasively, ultrasound topography. In the United States, prenatal screening began in 1966, and the number of identifiable disorders and metabolic defects continues to go. Testing of embryos prior to implantation in the uterus is one potential application of this technology, ensuring that embryos free of genetic diseases would be implanted (McCarrick).
Newborn screening takes place in early infancy, and involves the testing of blood or tissue for genetic diseases. The hope is that early intervention can help to avoid serious health problems or even mortality. Newborn screening is well established in the United States, with the first instance being the screening of newborns for phenylketonuria (PKU), a disorder that can be prevented by following a strict diet. Since that time, testing of African-American babies for sickle cell anemia and Ashkenazic Jews for Tay-Sachs disease have also become common (McCarrick).
Carrier screening identifies people that have a chromosome or genetic abnormality that may impact the person screen or their children. Blood or tissue samples are tested, and can identify changes in DNA or chromosomes, or the presence of a genetic trait that is seen in inherited diseases. Tests for sickle cell anemia and Tay Sachs disease are common, but tests for Huntington's disease, Duchenne muscular dystrophy, cystic fibrosis, hemophilia, and neurofibromatosis have become more common. Often, carrier screening is…
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