What Are Inherited Diseases? Types and Causes

Inherited diseases which are otherwise known as genetic disorders are diseases that occur as a result of an abnormality in the genetic makeup of an individual.

Generally, every individual is made up of genes and these genes carry an individuals identity and makeup. If there be any problems with the genes of that individual, there will be a physical manifestation which is known as a genetic disorder.

What exactly are genes?

Inherited Diseases

Genes are special fragments that are found within the DNA. These genes when they are read correctly after undergoing a process of transcription and translation to become proteins; can then provide the body with adequate instructions and pieces of information in order to allow the body function accurately

Research has shown that there about 22,000 genes that are found in a particular genome on a strand of DNA. Although the importance of genes cannot be overemphasized, however, they only make up a small percentage of all the DNA. Each individual has a specific number of chromosomes which are 23 in number.

On each chromosome are found a particular set of genes and they have specific locations where they are found. During fertilization and eventual fetal development, these genes are passed down to an offspring and as such we can say that the child inherits his genes from his parents.

There are two copies of each chromosome that are found in one individual and as such two copies of a particular gene.

During copulation and fertilization, each parent gives up one pair of their gene to their offspring and as such, every baby grows up with two sets of genes. These babies grow and pass on two sets of genes to their offspring and the cycle continues on.

The purpose of a gene is to carry out specific instructions for the body. Some genes do multitask by carrying out more than one instruction. These instructions that the gene carry out is mostly towards the formation of a particular protein and there are many types of proteins that carry out different functions. Proteins make up the building blocks of our bodies.

Having established the importance of genes, there are certain diseases that are caused as a result of the malformations of these genes and these diseases are known as inherited or genetic diseases.

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Sometimes these genetic malformations are passed down from one set of genes of the parent to the offspring and at other times, there is a spontaneous change in the DNA that affects the formation of these genes.

Due to the spontaneous change in the gene of the offspring, it is entirely possible to have a child with genetic disorders whereas the parents are healthy with no genetic malformations. This condition is known as a new mutation.

These new changes in the gene can cause the production of a protein which doesn’t function properly or the non-production of a protein which can irrevocably lead to inherited diseases.

For each inherited disease, the presentation of the genetic makeup is different. For some diseases, all the mistakes that are caused by a particular gene result in a single genetic disorder.

However, for other cases, the different changes on each of these genes can cause different health, developmental or mental problems. It can even give rise to different genetic disorders.

Typically, most genetic disorders are passed down from parent to offspring making them inherited and this inheritance can either be in a dominant or a recessive manner. Generally, we have 22 chromosomes which come along with two copies of each gene.

For females, they have two copies of all their genes on a single chromosome known as the X chromosome while for males, they have all their genes on two chromosomes which are the X and Y chromosome respectively.

Hence the males will have a single copy of the X chromosome genes and a single copy of the Y chromosome genes while females have two copies of the X chromosome genes.

For an inherited disease to be considered dominant, it means that the genetic disorder occurred on only one copy of the two genes. This gives the child a 50-50 probability of getting the disorder. The child may or may not get a genetic disease.

However, an inherited disease is said to be recessive when there are genetic abnormalities in both copies of the gene. This means that both parents must carry at least one copy of the infected gene in order for the offspring to be infected.

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Once both parents have a copy of an abnormal gene, there is a 25% chance that the child will inherit the two abnormal genes and such causing the child to have an inherited disease.

Research has shown that most parents are known as carriers and this is because they have one copy of an abnormal gene and one copy of a normal gene, hence the disorder is not expressed in them.

However, they carry the defective gene which will be passed on to their child hence increasing the chances of inherited diseases in their offspring.

Having understood what inherited diseases are and how they are passed on to their offspring, there are some types of inherited diseases that you must know about. They include

  • Chromosomal aberrations or abnormalities
  • Single gene defects
  • Multifactorial problems
  • Teratogenic disorders

Chromosomal aberrations or abnormalities

These abnormalities are diseases that are gotten as a result of malformations on any of the chromosomes.

These abnormalities may occur as a result of inheritance or as a result of a spontaneous change on the DNA of the offspring. This means that there may or may not be any family history that is related to this disorder. These disorders include

1. Aneuploidy

This occurs when a child has fewer or much more chromosomes than the normal number. Examples of aneuploidy include:

a. Down Syndrome

Down syndrome is one of the most common inherited diseases that we see all around us. This disease is caused by the abnormal duplication of the 21st chromosome in all or some of the cells of the body.

Generally, a human cell has a genetic makeup of 23 pairs of chromosomes. Down syndrome occurs when the 21st chromosome is copied three times instead of twice on a chromosome. When a person is diagnosed with down syndrome, the patient will most likely develop cognitive delays with a varying scale of severity.

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A child with down syndrome is also likely to develop congenital heart defects as well as smaller body stature, reduced muscle tone, and an upward slant found on the eyes. According to research, in the United States, about every 1 in 700 babies develop down syndrome.

Research has also found out that age is also a risk factor associated with down syndrome. This means that the higher in age the woman is at the time of birth, the higher the chances of the baby being delivered with down syndrome.

b. Turner syndrome

Turner syndrome is a disorder that affects majorly females. Turner syndrome is said to occur when the female is formed with one X chromosome or with one X chromosome and another partially formed X chromosome.

It has been established that a woman has two X chromosomes which must be fully formed. Once there is an absent or a partially formed X chromosome, then the child is said to have Turner’s syndrome.

There are two types of Turner syndrome which are the classical turner syndrome and the Mosaic Turner syndrome. The Classic Turner syndrome occurs when there is an absence of one of the X chromosomes while a Mosaic Turner syndrome occurs when there is the partial formation of one of the X chromosomes and as such some cells have a pair of X chromosomes while others have only a single strand of the X chromosome.

2. Deletion

The deletion occurs when a part of a particular chromosome is missing or a part of the DNA code is missing.

3. Inversion

Inversion occurs when a certain piece of the chromosome breaks off, turns upside down amd then reattaches itself. This phenomenon causes a malformation of proteins causing an inherited disease to occur.

4. Translocation

Translocation occurs when there is a rearrangement of a particular segment of a chromosome from one location to another and it could either be on the same chromosome or on different chromosomes. There are two types of translocation that can likely occur and they include;

a. Balanced translocation

This occurs when the rearrangement that occurs is balanced. This means that there is no DNA lost as there is an equal exchange on both chromosomes.

A parent who has a balanced translocation is seen to be healthy, however, the problem comes when she is pregnant as there is a risk of transferring unbalanced chromosomes during pregnancy and as such increase the child’s chances of inherited diseases.

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b. Robertsonian translation:

This occurs when one chromosome breaks off to join another chromosome.

Single Gene disorders

Single Gene disorders occur when a single gene is responsible for the entire genetic deformation.

It is also known as Mendelian Inheritance disorders. There is a higher risk of developing single-gene disorders and these diseases can be considered to be either dominant or recessive. Examples of this kind of disorders include:

a. Achondroplasia

This disorder is what usually causes dwarfism. It is a bone growth disorder that results in shortness of both the arms and the limbs. This disorder is caused mainly by spontaneous mutations of the FGFR3 gene and it is considered as an autosomal dominant disorder which means that only one parent is required to pass down this defective gene for the offspring to begin to show signs of this disease.

b. Marfan Syndrome

Mar fan Syndrome occurs as a result of a disorder of the connective tissues of the body. It is a rare case of inherited disease. People with Mar fan tend to grow tall with very long limbs and they usually develop cardigan problems as well as eye disorders. This disorder is also considered to be an autosomal dominant disorder.

c. Cystic fibrosis

Cystic fibrosis is a chronic genetic disorder that causes the patient to keep producing thick mucus which is sticky within their body systems. This, therefore, causes a disruption within their digestive tracts as well as their respiratory and reproductive systems respectively.

This disorder is an autosomal recessive disorder which is common. Research has shown that people who live with cystic fibrosis tend to develop more health problems which in turn reduces their life expectancy rates.

Another research has shown that 95% of men who have cystic fibrosis are seen to be sterile and they usually don’t live beyond 35 years of age. The mean age of people who live with cystic fibrosis has been estimated to be 33.4 years.

d. Sickle cell disease

Sickle cell disease is an autosomal recessive disorder that is mostly inherited and passed down from parent to offspring. Sickle cell anaemia occurs when the red blood cells are forced to change their shape from the normal round shape to a kind of sickle shape.

This change in shape then causes the blood to get clumped together and as such become caught within the blood vessels causing intense pain and serious complications such as damage of several organs, infections as well as acute respiratory disorders.

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e. Tay-Sachs disease

Tay-Sachs disease often occurs when there is a defect found on chromosome 15. Recall that for down syndrome, the problem occurs from the malformation on chromosome 21, however, for Tay-Sachs syndrome, the defect occurs in Chromosome 15.

This disease which is also considered to be an autosomal recessive disorder can become irreversibly fatal in children. Tay-Sachs causes a gradual decline in the nervous system of the body and as such, the child never gets to live beyond five years of age.

X-linked disorders

This kind of inherited disease often affects males. This disorder is developed as a result of aberrations of the X linked chromosomes.

This disease is often inherited by daughters who got the defective gene from their father’s and they end up becoming carriers with a 50% chance of passing it to their offspring. Examples of such disorders includes;

a. Duchenne Muscular Dystrophy

This is a rare disorder that mostly affects the male gender. This is an inherited disease that is characterized by progressive muscle weakness and muscle degeneration. It is usually seen to begin at the age of 4 and it tends to progress rapidly.

This disease is caused by the absence of a specific protein known as dystrophin which is responsible for keeping the muscle cells intact and the muscles in shape. This disorder is an autosomal recessive disease.

b. Haemophilia

Haemophilia is caused by the absence of a blood-clotting protein. It is characterized by the inability of the blood to clot and as such lead to excessive loss of blood. When the blood can’t clot properly, there will be both internal and external bleeding when there is a puny slight accident.

There are three types of haemophilia which are: haemophilia A which is caused by the deficiency of clotting factor VIII, haemophilia B which is caused by the deficiency of clotting factor IX and haemophilia C which is caused by the deficiency of clotting factor XI.

Among these three types of haemophilia, haemophilia A and haemophilia B are the most common types of haemophilia that are known.

Multifactorial diseases

These diseases occur as a result of a combination of environmental factors as well as gene mutations on multiple genes. They are also known as complex or polygenic diseases and they cause some chronic disease. Examples of such diseases caused by this type of inheritance include:

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a. Alzheimer’s disease

Alzheimer’s disease occurs as a result of a neurological disorder In which the cells of the brain begins to die off progressively thereby causing cognitive decline as well as memory loss.

It is a very common type of neurological disease and the causes can be attributed to genetic mutations.

Mitochondrial genetic inheritance

This type of genetic disorder is caused by the occurrence of mutations in the DNA of the mitochondria. Examples of such diseases include:

a. Leber’s Hereditary Optic Atrophy

This is a form of an inherited vision loss. It usually affects males much more than females. It usually starts with one eye before progressive to the other later on in life. Mostly, it has been seen to start between the ages of 10-30years.

b. Myoclonic Epilepsy with ragged red fibres

These occur as a result of brief shock-like jerks of muscle fibres. During an episode, the patient is often awake and able to see and recognise what goes on around him.

In conclusion:

For families that are at risk of developing the inherited disease, it is important to consult a certified genetic counsellor for genetic screening. These screenings can help determine the possibility of developing some of this diseases and also help you understand how to deal with the effects of a disorder, how to manage or treat it and likewise, how to prevent it if possible.

Inherited diseases can be painful especially to a parent watching a child go through pains without knowing what to do to stop it. If there is a possibility of prevention, one can only know via genetic screening.

Sources:

  1. Medicine Net
  2. Illumina
  3. Regiscollege
  4. IHTC
  5. AAO
  6. Mayo Clinic
  7. Epilepsy.com
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