The human genome is a complete set of instructions, just like a recipe book, administering growth and development. Nonetheless, the human genome can face mutations which can affect the individual bases (A, C, G or T) or much larger clumps of DNA or even chromosomes. DNA confers the code for making proteins, combination of amino acids performing most of the functions in body. When a part of DNA is altered in some way, the protein it codes for is also affected. As a result, it may not be able to perform its normal functions. Most probably, it depends on the location where these mutations occur; they may exert little or no effect, or may change the biology of cells of the body, resulting in a genetic disorder.
Any disease caused in whole or in part by an abnormality in the DNA sequence away from the normal sequence is referred to a genetic disease. The genetic disorders can range from minuscule to major. They may be the result of a mutation in one gene (monogenic disorder), mutations in multiple genes (multifactorial inheritance disorder), combination of gene mutations with the environmental factors like cigarette smoke and exposure to radiation, or any damage to chromosomes (alterations in the number or structure of entire chromosomes).
Kinds of Genetic Disorders
Genetic disorders can also include specific kinds of birth defects, developmental problems, sensory deficits, and chronic diseases that are inherited from one or both parents.
Following signs can indicate birth defects:
- Misshapen skull
- Congenital heart disease
- Diaphragmatic hernia
- Genital malformations
- Spina bifida or open spine defects
- Cleft lip or palate
- Missing fingers or toes
- Missing or incomplete arms or legs
These signs are indications of chronic diseases:
- Slow growth or short stature
- Childhood cancers
- Cystic fibrosis
- Kidney or urinary tract disease
- Bleeding disorders
- Sickle cell disease
These signs can indicate developmental problems:
- Mental illness
- Developmental delay
- Attention deficit/hyperactivity
- Failure to thrive
- Autism spectrum disorders
- Mental retardation
- Loss of developmental skills
- Learning disability
- Low muscle tone
- Speech problems
Following signs can indicate sensory deficits:
- Retinal problems
- Extreme nearsightedness
- Extreme farsightedness
- Hearing loss
Some of these signs can be same for conditions that are not inherited and may appear during childhood (learning problems or developmental delays) or at birth (congenital heart disease, cleft lip, or palate).
Major physical signs of genetic disorders
The following signs may indicate the possibilities of genetic disorders. However, these features can typically found in people without disorder. Consult your doctor when notice two of the following features:
- Ear abnormalities
- Unusually tall or short stature
- Unusually shaped eyes
- Different colored eyes
- Unusual birthmarks
- White patches of hair
- Large or small tongue
- Webbed fingers or toes
- Unusual body odor
- Excessive body hair
- Excessive skin
- Facial features that may be different from other family members
- Brittle or sparse hair
- Loose or stiff joints
- Misshapen teeth
- Missing or extra teeth
Four major types of genetic disorders
Different types of genetic disorders (probably inherited) may include:
- Single gene disorder
- Multifactorial disorder
- Mitochondrial disorder
- Chromosome abnormalities
Single gene inheritance disorders
Single mutated gene causes a single-gene disorder (or monogenic disorder), also known as Mendelian or monogenetic inheritance. It can be passed on to subsequent generations in multiple ways. Genomic imprinting and uniparental disomy can affect inheritance patterns. The cleavage between recessive and dominant kinds are not “hard and fast”, even though the cleavage between autosomal and X-linked types are.
There are hundreds of known single-gene disorders. Also, they have different patterns of genetic inheritance, including:
- Autosomal dominant inheritance is the type in which only one copy of a mutated gene (from either parent) is essential to cause the disease.
- Autosomal recessive inheritance is the type in which two copies of a mutated gene (one from each parent) are essential to cause the disease.
- X-linked inheritance is the type in which the mutated gene is present on the female or X-chromosome. X-linked inheritance can either be dominant or recessive.
Most congenital metabolic disorders are called inborn errors of metabolism that result from single-gene mutations. A lot of such single-gene disorders can lower the fitness of affected people and are thus present in the population in relative lower frequencies when compared to what would be expected and on the basis of simple probabilistic calculations.
The major examples of single-gene disorders are:
- Sickle cell anemia (sickle cell disease)
- Cystic fibrosis
- Huntington’s disease
- Alpha- and beta-thalassemias
- Marfan syndrome
- Fragile X syndrome
Most probably they refer to the complex diseases cause by changes in more than one gene, more often in a complex interaction with lifestyle and environmental factors like diet and cigarette smoke.
Multifactorial diseases usually tend to run in families, however the inheritance does not fit simple patterns like Mandelian diseases. Strong environmental components may be the cause of multifactorial diseases like blood pressure. Other factors may include:
- Inflammatory bowel disease
- Heart disease
- Cleft palate
- Autoimmune diseases such as multiple sclerosis
- Intellectual disability
- Mood disorder
- Refractive error
Multifactorial disorder usually clusters in families signaling they do not have clear patterns of inheritance. They are also difficult to study and handle as the certain factors causing most of these disorders have not yet been characterized. Several methodological can be used to check associations of genotype–phenotype in order to identify the cause of complex disorders. One method is the genotype-first approach which may start by identification of genetic variants and then determining the associated clinical manifestations. This is contrary to the traditional phenotype-first approach, and can specify causal factors that have initially been obscured by clinical heterogeneity, penetrance, and expressivity.
Examples of multifactorial inheritance include
- Heart disease
- Alzheimer’s disease
- High blood pressure
Multifactorial inheritance accounts for approximately 20 percent of congenital anomalies.
This type of disorder is also called as maternal disorder. It is quite rare and applies to the 13 genes encoded by the mitochondrial DNA. Only affected mothers can pass the defective genes of mitochondrial DNA to their children because only egg cells contribute mitochondria to the developing embryo.
A majority of mitochondrial diseases, specifically when symptoms develop in early life, are typically caused by a nuclear gene defect. This is because the mitochondria are mostly developed by non-mitochondrial DNA. Mitochondrial disorder often follows autosomal recessive inheritance.
The DNA mutations due to Mitochondrial disorder may cause the mitochondria fail to generate enough energy to sustain the body’s cells. This disease can affect any organ or part of the body. The symptoms depend on the part of the body the disorder affects. A few common symptoms may include:
- Developmental delays
- Poor growth
- Autism spectrum disorder
- Muscle weakness
- Visual problems
- Heart, liver, or kidney disease
- Hearing problems
- Intellectual disabilities
- Loss of muscle coordination
- Respiratory disorders
Currently there is no highly effective treatment for mitochondrial genetic disorder. But nutritional management, vitamin supplements, amino acid supplements, and medications to help treat specific issues like muscle weakness or seizures, can reduce the symptoms.
Examples of this type of disorder are:
- Sensorineural deafness
- Pigmentary retinopathy
- Diabetes mellitus
- Proximal myopathy and exercise intolerance
- External ophthalmoplegia
- Leber’s hereditary optic neuropathy
Chromosomal abnormalities may be the result of a chromosome that is missing, extra, or having a chromosome that has some kind of structural abnormality. It can be from an irregular portion of chromosomal DNA.
Chromosomal abnormalities usually develop when there is a defect in cell division. Usually these defects occur within the egg or sperm, but they can also develop after conception. It is also possible to inherit a disease directly from a parent however; some people may develop for the first time.
The common types of chromosomal disorders can be described in the following way:
- Aneuploidy-there may be the wrong number of chromosomes (too many or too few). Examples of an extra chromosome are Trisomies 13, 18 and 21 are. Turner syndrome occurs when there are too few chromosomes. In this disorder, one of the sex chromosomes is not transferred, so a single X chromosome, or 45 rather than the usual 46 is left.
- Deletion– a part of a chromosome is not present which indicates that the genetic material on the missing part of chromosome is also absent. Cri du Chat syndrome is a condition in which a portion of chromosome 5 is deleted.
- Inversion– refers to a condition in which chromosome suffer breakage. The broken piece may try to reattaches itself turning upside down. This disorder may or may not result in birth defects; it usually depends on their exact structure.
- Translocation– reshuffling of a segment of a chromosome from one location to another. A balanced reshuffling may has an equal exchange which means no genetic material is added or lost. For a person with this genetic disorder, however, there may be the risk to their offspring. There is a possibility that the chromosome may be transferred as an unbalanced chromosome which indicates genetic material can add or lost causing a variety of complexities in varying degrees for the offspring.
- Mosaicism– indicates the presence of two or more chromosomal patterns in the cells of a person which results in two or more cell lines. In this case, some cells will have 46 chromosomes and some cells will have other than 46 (either less than or greater than 46).
These chromosomal defects account for only about 6% of all the birth defects.
The list below gives some examples of chromosomal abnormalities.
- Down syndrome
- Wolf-Hirschhorn syndrome
- WAGR syndrome
Identification of genetic disorders
In case of a family history of a genetic disorder, considering genetic counseling is important in order to check whether genetic testing is appropriate for the diseased or not. Lab tests can clarify if gene mutations are responsible for that condition. Important thing to consider is that carrying the defective gene does not always mean ending up with it. Genetic counselors may explain the condition better by protecting your health by taking essential steps.
DNA testing for genetic disorders, in case of a family history, can be a significant part of starting a family. A few courses of action may include:
- Prenatal diagnostic testing: To find out whether an unborn child suffers a higher risk of the specific genetic disorders. Prenatal diagnostic testing utilizes a sample of fluid from the womb (amniocentesis).
- Carrier testing: This shows either you or your partner has mutated gene/genes linked to any genetic disorders. Doctors suggest this test for everyone considering pregnancy, even if there is no family history.
- Prenatal screening: It usually involves testing of blood from a pregnant woman that shows the chances of having a common chromosome condition of an unborn child.
Treatment for genetic disorders
Most of the genetic disorders do not follow a proper cure. While some have treatments that may slow the rate of disease progression or just lower their impact on life. The type of treatment, however, depends on the type and severity of the genetic disorder. Try to catch complications early with the help of medical surveillance, like:
- Blood transfusion to manage the levels of healthy blood cells.
- Certain treatments like radiation therapy for cancer.
- Medicines to manage disease symptoms or chemotherapy to slow the growth of abnormal cells.
- Physical, occupational or speech therapy to enhance abilities.
- Surgery for abnormal structures or to treat other complications.
- Organ transplant to replace a nonfunctioning organ with one from a healthy donor.
- Nutrition counseling or dietary supplements may help get the right nutrients for body during disease.
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