Genetics is the scientific study of how specific traits and physical characteristic are passed down from parents to their children. This is also known as heredity.
In the mid-1800s, a monk named Gregor Mendel selectively cross-bred the common pea plant (Pisum sativum) to learn more about heredity. He bred plants that were homozygous for different pea colors. This means each plant contained two identical alleles, which are variations of a single gene. Each gene contains two alleles because one is inherited from each parent.
For example, a red pea plant (expressing two dominant RR alleles) and a white pea plant (expressing two recessive ww alleles) were bred to produce offspring. The genes of the first group of offspring were heterozygous. This means the plants inherited two different alleles (Rw) for the single gene that expresses color.
Genotype refers to the individual's genetic makeup. Phenotype refers to the physical characteristics or traits that an individual expresses.
After years of research, Mendel discovered that certain physical traits appear in offspring without any blending of parental characteristics. For instance, if a red pea plant and a white pea plant are bred, their offspring do not express pink flowers. Instead, they produce either red or white flowers. This discovery was important because the leading theory in biology at the time was that inherited traits blend from generation to generation.
The cross-bred pea plants did not express intermediate colors (like pink) because out of the two inherited alleles, one is dominant over the other one. If plants have two different alleles, the dominant one prevents the other one from expressing physical characteristics. For instance, if red is the dominant allele and white is the recessive, plants that have the genotype Rw will be red.
Therefore, even though the "white" allele is not expressed in the pea plant flower, the trait can still be passed down to the next generation. The dominant red allele does not alter the recessive white gene, and both alleles have an equal chance of being passed on to the next generation.
Principle of segregation: Mendel developed the principle of segregation, which states that for each trait, the pair of alleles from each parent separates and the offspring only inherits one allele from each parent. Each allele is inherited by chance.
Principle of independent assortment: According to Mendel's principle of independent assortment, different pairs of alleles are passed to offspring independently of one other. Therefore, new combinations of genes, which are not present in either parent, are possible in the offspring.
Nearly every cell in the body contains a nucleus, which contains two sets of chromosomes. These chromosomes contain the building blocks of life called DNA (deoxyribonucleic acid). Each parent provides one set (23) to his or her offspring. Therefore, each person has 23 pairs of chromosomes. The X and Y chromosome are called sex-determining chromosomes because they distinguish males from females. Human females have a pair of X chromosomes, while males have one X and one Y chromosome.
Chromosomes can be seen under an ordinary light microscope. Therefore, some hereditary defects, such as Down's syndrome, can be detected after a microscopic analysis. However, most genetic diseases can only be diagnosed after a more sophisticated analysis that involves studying DNA sequences within the genes.
James Watson and Francis Crick first discovered the molecular structure of DNA in 1953. DNA contains the genetic makeup of an individual. Genes control the functioning of the cells by instructing cells to make new molecules (usually proteins).
DNA is a large molecule that looks like a twisted ladder. This unique shape is called a double helix. The sides of the double helix are made of alternating sugar and phosphate molecules. The rungs of the "ladder" are made of smaller molecules that contain nitrogen. These molecules include adenine, thymine, cytosine, and guanine. The sequence of these molecules provides the genetic code within the genes that control the development, growth, and functioning of all the cells in the body.
A single gene is made of a specific sequence of nitrogen molecules within a DNA molecule. Each human is made up of about 30,000 genes. The total genetic composition of an individual is called the complete human genome.
The DNA of the activated gene uncoils and exposes instructions for the production of a new messenger molecule called messenger RNA (mRNA). This chemical then incorporates the identical code of the exposed DNA. The mRNA then leaves the nucleus of the cell and enters the body of the cell. Here, RNA controls the production of a new molecule, based on the code it is carrying.
Sometimes mistakes in this process occur, which may cause mutations and lead to disease. Mutations cannot be fixed. However, not all mutations are detrimental.
A genetic disease or disorder is a condition that is caused by an abnormal expression of one or more genes. This occurs when the chemicals that make up an individual's genes are incorrectly deleted, added or substituted. If the mutation causes the cells in the body to stop functioning properly, the person may develop a disease or disorder.
Some patients may carry the mutated gene, but they do not experience symptoms of the disease or disorder. This occurs when the mutated gene is inherited as recessive trait. In order to develop an autosomal recessive disorder, one mutated gene from each parent must be inherited. Individuals who only have one mutated gene and do not express symptoms are called carriers.
There are thousands of known genetic disorders. Most disorders are rare and only affect one person out of several thousand or more. Cystic fibrosis is one of the most common genetic disorders, with about five percent of the U.S. population carrying at least one copy of the defective gene.
There are four different types of genetic disorders - single-gene, multifactorial, chromosomal, and mitochondrial.
Single-gene disorders: Single-gene disorders, also called Mendelian or monogenic disorders, are caused by mutations in the DNA sequence of one gene. Genes code (provide instructions) for proteins, the molecules that perform most life functions. When a gene is mutated, the protein it codes for no longer functions properly, and a disorder may result. Currently, there are more than 6,000 known single-gene disorders. Researchers estimate that about one out of every 200 people are born with single-gene disorders. Some examples include cystic fibrosis, sickle cell anemia, Marfan syndrome, and Huntington's disease.
Single-gene disorders are inherited as autosomal dominant, autosomal recessive, or X-linked recessive traits.
If a disorder is autosomal dominant, an individual will have the disorder if he/she carriers one dominant allele. Individuals who have the disease have a 50% of passing the disease on to each of their children.
In order to inherit an autosomal recessive disorder, one mutated gene from each parent must be inherited. Individuals who only have one mutated gene and do not express symptoms are called carriers. Carriers have a 50% chance of passing the abnormal gene to each of their children. If both parents are carriers, there is a 25% chance that each of their children will inherit the disease and a 50% chance that each of their children will be a carrier.
If the disorder is X-linked recessive, it will affect males almost exclusively. If a male inherits the mutated gene, he will develop the disease because he has only one X chromosome. Females, on the other hand, have two X chromosomes. The female would need to inherit two mutated X chromosomes in order to develop the disease. However, if the female inherits one mutated gene she is a carrier for the disease, and there is a 50% chance she will pass the gene to each of her children. Carriers do not express symptoms of the disease.
Multifactorial disorders: Multifactorial disorders are caused by a combination of environmental factors and mutations in multiple genes. Genetic mutations alone do not necessarily result in multifactorial disorders. Having particular genetic mutations simply makes patients more susceptible to developing a disorder. For instance, different genes that influence breast cancer susceptibility have been found on a variety of chromosomes. Some of the most common chronic (long-lasting) disorders are multifactorial disorders. Examples include heart disease, obesity, Alzheimer's disease, high blood pressure, arthritis, diabetes, and cancer.
Chromosomal disorders: Since chromosomes contain genetic material, abnormalities in their structures may lead to disease. Some chromosomal disorders occur when there are missing or extra copies of chromosomes, while others occur when chromosomes break and rejoin at the wrong locations. Some types of chromosomal disorders can be detected after a microscopic examination. For instance, Down's syndrome (trisomy 21) is a common disorder that occurs when a patient has three copies of chromosome 21, instead of just two.
Mitochondrial disorders: Mitochondrial disorders are caused by mutations in the nonchromosomal DNA of mitochondria. Mitochondria are small rod-like organelles (organ-like structures within cells) that provide energy to the cells. Each mitochondrion may contain 5-10 pieces of DNA. Some of the most common mitochondrial disorders include Lever's hereditary optic atrophy (eye disease), a type of epilepsy called MERRF (myoclonus epilepsy with ragged red fibers), lactic acidosis, and a type of dementia called MELAS (mitochondrial encephalopathy).
Genetic tests are used to confirm a diagnosis of patients who have symptoms of a genetic disorder.
They are also used to determine whether an individual is a carrier for a gene. While carriers do not express the disease, they may pass the disorder on to their children.
The tests can be used to screen embryos for disease before implanting them in a woman's uterus.
Genetic testing for adult-onset disorders like Huntington's disease are controversial because they are conducted in healthy individuals who are at high risk for the disease, but do not express symptoms. They have sparked debate because these tests only provide a probability for developing the disorder. Some people who carry a disease-associated mutation may never develop the disease. Researchers believe that in some cases, other factors, such as unknown genetic mutation or environmental factors, may have an impact on whether the individual actually develops the disease.
Pregnant mothers may have their unborn children tested for genetic disorders. In order to retrieve a sample of the fetus' cells for testing, amniocentesis or chorionic villus sampling may be performed. During amniocentesis, a long, thin needle is inserted into the pregnant woman's abdominal wall to the uterus. A small amount of fluid is removed from the sac surrounding the fetus. During chorionic villus sampling (CVS), a small piece of tissue (chorionic villi) is removed from the placenta. There are risks associated with these procedures, including miscarriage. Patients should discuss the potential risks and benefits of these procedures with their healthcare providers.
Gene tests (also called DNA-based tests) are the newest and most accurate techniques used to detect genetic disorders. A gene test analyzes the DNA molecule itself. Other genetic tests include biochemical tests (for gene products such as enzymes and proteins) and microscopic examination of chromosomes. During a chromosome analysis, the chromosomes are visible after they are treated with special chemicals in a process called staining. A DNA sample can be taken from any body tissue or from blood.
Genetic testing laboratories currently offer more than 900 genetic tests. However, there are no established regulations for evaluating the accuracy and reliability of genetic testing. Most genetic tests are categorized as services that the U.S. Food and Drug Administration (FDA) does not regulate.
Genetic tests can cost anywhere from $200 to $3,000. In most cases, health insurance companies do not cover the test. Those insurance companies that do will have access to the test results. However, in April 2008, the U.S. Senate passed the Genetic Information Nondiscrimination Act (GINA), an amended version of H.R. 493, which passed the House April 25, 2007. The bill prohibits employers and health insurance companies from using the results of predictive genetic tests to discriminate against their workers or members.
The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.