Genetic Disorders: Definition, Causes, List Of Rare & Common Diseases

The blueprint for living organisms is contained in the _genetic code_ found in the nuclei of cells. The DNA double helix molecules of the chromosomes consist of encoded instructions allowing cells to produce the proteins and other substances necessary for life.

When the DNA is damaged or has mistakes in the code, cells can't produce some of the materials needed, or they produce the wrong kind, causing genetic disorders, genetic disease or special genetic conditions.

Such genetic disorders can have many causes.

DNA molecules can be damaged by environmental factors, or they can replicate incorrectly during cell division. Some genetic conditions are inherited while others develop due to internal factors and influences from lifestyle or exposure to toxins or radiation.

Sometimes the mistake is tiny with just one coded element out of place while in other cases whole chromosomes may be missing. In each case, a specific genetic disorder or genetic disease results.

A genetic disorder is an abnormal condition caused by an error in the genetic code. The DNA sequences making up the genome of an organism must be completely correct, or the biological processes that rely on the encoded instructions will not work properly. Some errors are not important, but a few cause the most common genetic diseases, and many can be responsible for rare genetic conditions.

Cells often check their DNA for errors, especially before dividing. These safety measures mean that even the most common genetic disorders are comparatively rare, but their occurrence in the general population means that the DNA checks and redundancy are not foolproof.

A genetic abnormality means that when the DNA sequence containing the error is read, the wrong instructions are carried out. If the cell realizes there is a problem, the material encoded in the sequences may not be produced at all. If duplicates exist or extra chromosomes are present, excess material may be produced.

Typical genetic abnormalities include:

• A single nucleotide in a DNA sequence may be missing or misformed.
• Single nucleotides or DNA sequences may be duplicated.
• Parts of a DNA sequence may be missing.
• Chromosomes may be misformed.
• Entire chromosomes may be missing.
• An organism may have an extra chromosome.

When a gene is used to give the cell instructions on how to produce a needed organic compound, a transcription mechanism makes an RNA copy of the gene. The RNA copy leaves the nucleus and uses the cell organelles such as ribosomes to synthesize the organic compound.

When there is a simple error, the error may be copied, but the needed substance is not produced. If a sequence or chromosome is missing, the transcription mechanism can't find the sequence it is looking for. If there is duplication or extra DNA, the transcription mechanism may produce extra copies. In each case, the abnormal production of proteins, hormones and enzymes leads to genetic disorders.

The genetic abnormalities that cause genetic disorders range from single-gene errors where only one gene is abnormal, to complex, multifactorial disorders that have many influencing abnormalities.

Some genetic diseases are single-gene disorders caused by a simple mistake in the genetic code. The cause of these diseases can often be traced back to the source gene, but the causes of other genetic diseases are so complex that finding the complete pattern of genetic abnormality is challenging.

The causes of these genetic abnormalities include:

• Inheritance. A genetic abnormality may be passed down from parents to offspring.
• **Gene Mutations.** A change in DNA sequences may happen spontaneously during cell division or due to external factors such as drugs or chemicals.
• Damage. DNA sequences may be disrupted by environmental factors such as radiation or toxins.
• **Mitosis.** Chromosomes may not separate properly, causing missing sequences or chromosome segments.
• **Meiosis.** During the production of sperm and egg cells, the chromosomes are distributed unevenly, resulting in extra or missing chromosomes in the fertilized egg.

The causes of genetic abnormalities can be separated into two classes: inherited defects and defects causes by environmental and behavioral influences. The latter can include effects such as pollution or lifestyle effects such as smoking, drug use and diet. These effects can accumulate as an organism ages.

• Down syndrome. The chromosomal disorder has three copies of chromosome 21, called trisomy 21. It results in intellectual disability with characteristic folds around the eyes, and flatter and rounder faces.
• Cystic fibrosis. The condition is due to a defective single gene, the CFTR gene on chromosome 7. The defective gene results in the production of excessively viscous mucous secretions, causing problems in the lungs.
• Klinefelter syndrome. The syndrome is due to an extra X chromosome in males, giving XXY chromosomes. The defect results in small testicles, infertility and mild developmental delays.
• Sickle cell disease. Inherited mutations in the genes for hemoglobin result in blood cells that have a narrow, sickle shape rather than the normal round shape. The condition results in shortness of breath but may also cause resistance to malaria.
• Huntington's disease. A chromosomal defect on chromosome 4 triggers early and progressive dementia.
• Heart defects and disease. The condition is made up of a multifactorial disease group that may have inherited genetic components as well as environmental and lifestyle influences.
• Fragile X syndrome. Duplicate sequences in the X chromosome result in learning disabilities.
• Hemophilia. Inherited defective genes on the X chromosome lead to a deficiency in clotting factors in the blood. Hemophiliacs may have excessive bleeding from even minor cuts.

• Breast cancer gene. Inherited mutations in the BRCA1 and BRCA2 genes affect the production of tumor suppressor proteins and increase the risk of breast cancer.
• Larsen syndrome. Mutation of the FLNB gene affects collagen formation and results in abnormal bone growth.
• Osteogenesis imperfecta. A defect in a gene that produces collagen is sometimes inherited and sometimes developed spontaneously. It causes brittle bones.
• Proteus syndrome. A mutation in the AKT1 gene overproduces growth factor enzymes that lead to abnormally high growth rates in skin, bones and some tissues.
• Marfan syndrome. An inherited defective gene responsible for the production of fibrillin causes faulty connective tissue affecting the eyes, skin, heart, nervous system and lungs.
• Turner syndrome. A missing or partly absent X chromosome in women leads to the development of special facial characteristics, low growth and sterility.
• Tay-Sachs disease. A mutated HEXA gene responsible for an enzyme critical for brain and nerve cell development results in progressive deterioration of mental and muscle control functions.
• SCID (Severe Combined Immunodeficiency). A group of up to 13 mutated genes affect the immune response, making people who have the disorder susceptible to infections.

Many of the genetic disorders can be diagnosed but not treated because the functions of the underlying genes are not completely known. When treatment is possible, the organic substances normally produced by the defective genes may be replaced, and patients can lead more normal lives. The relatively new field of gene therapy explores and continuously adds new research to this kind of treatment.