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, radiation, or viruses. Negative mutations can lead to cancer, and often result in the death or malfunctioning of the...
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