Each cell within the human body contains DNA, made up of a sequence of billions of chemicals called base pairs. The sequence of specific base pairs codes for thousands of genes found on two sets of 23 chromosomes--large pieces of DNA, each containing several genes. These genes form the blueprint for protein synthesis that supports cell and organ functioning and ultimately support life. There are a number of genetic diseases, which range greatly in their symptoms and severity, all ultimately caused by genetic mutations and abnormalities.
Inherited Mutations
Many genetic diseases are caused by mutations passed down through families. Inherited mutations can be classified as autosomal dominant or autosomal recessive. An autosomal dominant mutation requires that only one mutant gene be passed onto the child from the mother or father, and only occurs if one or both parents have the resulting genetic disease. Haemophilia, a blood disorder characterized by an inability to form blood clots, is an example of an autosomal dominant genetic disease.
Autosomal recessive genetic diseases require that both copies of the inherited gene carry mutations. Mutations that cause autosomal recessive genetic diseases may be passed on by carriers--people who do not have the genetic disease, but have one mutant gene. If both parents carry a genetic mutation, their children have a one in four chance of developing the genetic disease. Cystic fibrosis, a deadly condition characterized by excess mucus production, is an autosomal recessive genetic disease.
Spontaneous Chromosomal Abnormalities
In some cases, children may develop genetic diseases that are not inherited, but occur as a result of spontaneous genetic abnormalities found in the egg or sperm during conception. Normal egg and sperm cells partially develop through a process called meiosis, which results in cells that contain one set of 23 chromosomes. When the egg and sperm fuse together, they create a cell with two sets of 23 chromosomes that can develop normally. In some cases, defects in meiosis can lead to the development of an egg or sperm carrying an extra copy of one chromosome, so that when the egg and sperm fuse they create a cell with 47 chromosomes.
Genetic diseases caused by these chromosomal are typically not inherited, and can often occur without a family history of the genetic disease. Down syndrome, a genetic disease that leads to mental retardation and learning disabilities, is caused when a child inherits an extra copy of chromosome 21. The Children's Hospital of Pittsburgh indicates that 95 percent of Down Syndrome cases occur due to spontaneous, not inherited, genetic abnormalities.
Acquired Mutations
In some cases, genetic diseases may be caused by acquired mutations. Such is the case for many forms cancer, the most common genetic disease, according to The Human Genome. Acquired genetic mutations often occur as a result of cellular mistakes that occur during cell division. During each cell division, the cell must copy a sequence of billions of nucleotide bases that make up DNA. Any uncorrected mistakes in copying the DNA are passed on to daughter cells, and the accumulation of multiple genetic mutations can lead to defects in cell behavior, ultimately leading to cancer.
Genetic diseases caused by acquired mutations may also occur due to exposure to mutagens, such as chemicals or radiation. Some chemicals, such as benzene, can enter the cells and increase the number of genetic mutations, while exposure to radiation can directly damage DNA and lead to mutation. Limiting exposure to known mutagens can decrease the rate of new mutation, and decrease the risk of genetic diseases such as cancer.
