A genetic disorder is a condition that occurs as a result of a mutation to DNA. There are several different genetic disorders.
Most cells within the body contain the molecule DNA. This molecule provides the cell with instructions on how to function. A change or mutation within the DNA can cause the cell to function abnormally.
This article outlines what genetic disorders are. It discusses the different types, the main symptoms of each type, and how they develop.
Genetic disorders are conditions that occur as a result of changes to or mutations in DNA within the body’s cells.
Most cells in the body contain long strands of DNA that provide the cell with instructions. Each DNA strand is tightly coiled around a protein called a histone. This coiled structure is called a chromosome.
Chromosomes contain small sections of DNA called genes. These genes provide the body with a specific set of instructions. Each human cell normally contains 23 pairs of chromosomes, with one of each pair provided by each parent. Therefore, a person has two copies of every gene.
A change or fault in the DNA can cause a genetic condition. Since genes pass from parent to child, these disorders may be heritable. However, not everyone with a genetic condition in their family will experience symptoms of the disorder.
Genetic conditions can affect any gene or chromosome. This means that there are a wide range of genetic disorders, each causing various symptoms.
The human genome refers to all the genes and DNA necessary to build and maintain a human.
The Human Genome Project (HGP) was a global research project to map the human genome. The project established the sequence of the human genome and the function of different genes.
The HGP estimated there to be around 20,000–25,000 genes in the human genome. The DNA inside these genes contain four chemical bases that act as units of information. They are:
Each DNA molecule contains two twisted strands of DNA. Pairs of chemical bases connect one DNA strand to the other, resembling the rungs of a ladder. The base pairs of chemicals between each strand always combine in a specific way. For example, adenine on one DNA strand always pairs with thymine on the opposite DNA strand.
The order of the chemical base pairs in each molecule of DNA affects what instructions the DNA provides to the body. DNA sequencing involves reading the order of these base pairs.
Sequencing the human genome was an important step in understanding how genes can cause disease.
Genetic conditions tend to run in families. Parents pass genes on to their children, and some of these genes may contain the basis of a genetic disorder.
However, each parent only passes down half of their genes. The version of each gene that a parent passes down is known as an allele.
If two alleles from each parent differ, the body may only take instructions from one of them. The allele that the cell takes instructions from is known as the dominant allele. The other is known as the recessive allele.
Some genetic conditions are carried by a dominant allele, while others are carried by a recessive allele. Generally, a person will only inherit a particular genetic disorder if they have at least one dominant allele for the disorder or two recessive alleles for the disorder.
A single inheritance, or monogenic, disorder is a condition that results from a fault within a single gene.
The sections below will outline some examples of single inheritance conditions.
Huntington’s disease is a degenerative brain disorder that causes:
- uncontrolled movements
- emotional disturbances
- cognitive decline
Huntington’s disease develops due to a mutation on a dominant allele within chromosome 4. People with this allele will eventually develop the condition.
There is currently no way to stop or slow the progression of Huntington’s disease.
However, certain medications may help a person manage their symptoms. These include medications to help control involuntary movements and medications to treat mood shifts, irritability, and depression.
Sickle cell diseases
Sickle cell anemia is a type of SCD in which the red blood cells that transport oxygen to the body’s tissues are misshapen. Their unusual shape means that they are less able to transport oxygen and more likely to clump together.
Clumps of these blood cells may block a blood vessel, potentially causing:
- acute chest syndrome
SCDs occur as a result of mutations in the HBB gene. This gene provides instructions for the production of red blood cells.
SCDs are recessive. This means that a person would need to inherit two alleles containing the mutation in order to have the disorder.
Treatments for SCDs aim to prevent complications and prolong life.
A doctor may prescribe the medication hydroxyurea to increase the size of red blood cells, thereby increasing the amount of oxygen each cell can transport.
Muscular dystrophies are X-linked disorders, meaning that they affect a gene on the X chromosome. These conditions are more common in males. This is because males have one X chromosome and one Y chromosome, whereas females have two X chromosomes. In females, the unaffected X chromosome can counteract the affected one, but in males, there is not another X chromosome to do this.
There is currently no treatment available to stop or reverse muscular dystrophies.
Instead, treatment aims to prevent complications and improve a person’s quality of life. Examples of such treatments include:
- physical therapy, to help maintain muscle strength and flexibility
- respiratory therapy, to help maintain the strength of the respiratory muscles
- speech therapy, for people in whom a weakness of the throat or facial muscles affects speech
- occupational therapy, to help a person use assistive devices such as wheelchairs
- one or more of the following medications, to help slow or control symptoms:
- glucocorticoids, to increase muscle strength and slow the progression of muscle weakness
- immunosuppressants, which may help delay damage to muscle cells
- anticonvulsants, to help control muscle spasms and seizures
- antibiotics, to treat respiratory infections
Multifactorial inheritance disorders (MIDs) are conditions that develop due to a combination of genetic factors and environmental or lifestyle factors.
Some of these non-genetic factors may include:
- drinking alcohol
- eating an unhealthful diet
- not getting enough sleep
- living in an area that has high levels of air pollution
Some conditions that may fall into the category of MIDs include:
Certain genetic mutations can increase the risk of these conditions. However, there is no clear pattern of inheritance.
Chromosomal abnormalities are problems that affect a chromosome. Chromosomal abnormalities can involve:
- having a missing chromosome
- having an extra chromosome
- having a chromosome that has some kind of structural abnormality
Chromosomal abnormalities usually occur when there is an error as a cell is dividing. These errors usually occur within the egg or sperm, but they can also happen after conception.
It is possible to inherit a chromosomal abnormality from a parent. However, some develop within a person for the first time.
The sections below will outline some examples of chromosomal abnormalities.
Down syndrome is a type of chromosomal abnormality that affects intellectual and physical development.
Down syndrome occurs when a person receives an extra copy of chromosome 21. This means that each cell within the body contains three copies of chromosome 21 instead of the usual two copies.
Down syndrome is a lifelong condition. However, various types of therapy can help with a person’s intellectual and physical development. Examples include:
- receiving extra help or attention at school
- speech therapy
- physical therapy
- occupational therapy
Wolf-Hirschhorn syndrome is a chromosomal abnormality that can affect the entire body. The major features of this condition include:
- delayed growth and development
- reduced muscle tone
- intellectual disabilities
Wolf-Hirschhorn syndrome develops due to a deletion of a section of chromosome 4. Most cases occur for the first time within the person who has the disorder. However, it is also possible for a person to inherit the condition from a parent who has the chromosomal abnormality.
There is currently no cure for Wolf-Hirschhorn syndrome. However, the following treatments may help a person manage their symptoms and improve their quality of life:
- physical or occupational therapy
- drugs that can help with specific symptoms, such as seizures
Mitochondria are biological structures that exist inside the body’s cells. They generate most of the energy that the cells need to carry out their biochemical reactions.
Mitochondrial disorders are a group of genetic conditions that affect DNA within the mitochondria themselves. These DNA mutations result in the mitochondria failing to produce enough energy to sustain the body’s cells.
Mitochondrial disorders can affect any organ or part of the body. The symptoms a person experiences will depend on the part of the body the disorder affects.
Some possible symptoms of mitochondrial disorders include:
- poor growth
- muscle weakness
- loss of muscle coordination
- visual problems
- hearing problems
- developmental delays
- intellectual disabilities
- autism spectrum disorder
- heart, liver, or kidney disease
- respiratory disorders
Mutations in mitochondrial DNA are inherited maternally. This means that only a mother can pass down mitochondrial disorders.
There is currently no cure or highly effective treatment for mitochondrial disorders.
However, the following treatments may help a person manage them:
- nutritional management
- vitamin supplements
- amino acid supplements
- medications that help treat specific issues, such as muscle weakness or seizures
Genetic disorders occur as a result of a mutation to DNA. This mutation may affect whole chromosomes or the specific genes within chromosomes. DNA mutations may also happen within the DNA of mitochondria, which power a person’s cells.
Most genetic conditions are heritable, but some can occur for the first time within the person who experiences the disorder.
Genetic disorders are lifelong conditions. For this reason, treatments tend to focus on helping a person manage the symptoms, preventing complications, and improving quality of life.
In some cases, there may be medications available to help slow the progression of a particular disease.