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Inherited Disorders

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Inherited Disorders

We inherit most of our traits from our parents, like eye and hair colour. But these are not the only things we can inherit. Some diseases or disorders are caused by abnormalities or mutations in our DNA and they can be passed on from parent to child. These are known as inherited disorders.

Inherited Disorders Definition

Our genome plays a vital role in pretty much all health characteristics. However, there are some health conditions where the cause is almost exclusively due to a genetic abnormality or mutation. These are known as inherited diseases or genetic disorders.

The genome is all the genetic information of an organism, both coding and non-coding regions, and is composed of sequences of DNA nucleotides.

Mutation

Mutations are essentially abnormalities and changes in the genome. They may occur due to a mistake during DNA replication and cell division. Mutations can also occur due to environmental factors such as UV light exposure or mutagenic chemicals.

A mutagenic chemical is a chemical agent capable of inducing changes in DNA, such as tobacco.

There are various ways of classifying mutations. Concerning changes in DNA structure, mutations can be classified into small-scale or large-scale.

Small-scale mutations are changes in one or more nucleotides that only affect a single gene.

A gene is a DNA sequence that encodes either RNA or protein production.

Small-scale mutations can be further broken down into three types:

  • Substitution - occurs when one base nucleotide is changed to another.
    • For example, T is changed to C in the DNA structure of the gene.
  • Deletion - describe the removal of one or more nucleotides.
  • Insertion - is when one or more additional nucleotides are added to the gene.

Large-scale mutations (also known as chromosomal mutations) involve changes in a long segment of DNA at a chromosomal level. Therefore, these mutations often affect multiple genes.

These types of mutations can be further classified into categories involving deletions, insertions, translocations, or inversions of chromosome sections.

Our genetic make-up comprises the genes we inherit from our biological parents. Human cells have 46 chromosomes that are in pairs. They include 22 pairs of autosomal chromosomes and two sex chromosomes (either XX or XY).

We inherit half of our chromosomes from our biological father and the other half from our biological mother. Therefore, if there is a mutation in the parents' genome, there is the possibility that the mutation can be passed on to their children.

Note: not ALL mutations are inherited from our parents. Sometimes mutations occur for the first time due to DNA synthesis mistakes or exposure to mutagenic agents.

Types of Inherited Disorders

There are various ways to classify inherited disorders. This is often done according to the type of mutation that causes the disease. Here are three ways genetic disorders can be classified.

Recessive vs Dominant

Having a mutation in our genes does not always lead to disease. We mentioned we have 23 pairs of chromosomes. Furthermore, a gene can have different versions. The different variants of genes are called alleles.

Therefore, we can have two different alleles for every gene. Sometimes, both of these alleles must be mutated to cause disease. When a mutation is only present in one of the genes, the healthy allele on the other gene can compensate for the mutated one. These are called recessive mutations.

On the other hand, some mutations are dominant, meaning that the presence of the mutated allele in only a single copy of the genes can lead to a genetic disorder.

Monogenic vs Chromosomal vs Complex

Inherited disorders can be classified into three groups regarding the effect of the mutation on the genome structure and the elements other than the mutation that contribute to the disease. These are:

  • Monogenic or single-gene: As the name suggests, these genetic disorders result from a single gene mutation.

    • Example: sickle cell disease and cystic fibrosis

  • Chromosomal: Describes diseases that are caused by chromosomal mutations. Since these mutations affect multiple genes, they are often more challenging to treat.

    • Example: Down syndrome

  • Complex: A combination of gene mutations and other factors, such as diet and chemical exposure, are responsible for the disease. These disorders are also called multifactorial.

Sex-linked vs Autosomal

We mentioned earlier that humans have 22 pairs of autosomes and two sex chromosomes, either XX in females or XY in males.

  • Autosomal genetic disorders are those caused by mutations in genes on autosomal chromosomes.

  • Sex-linked disorders, however, describe genetic disorders due to mutations on the sex chromosomes, most commonly on the X chromosome.
    • Examples of X-linked genetic disorders include Duchenne muscular dystrophy and haemophilia.

It is important to note that since men have only one X chromosome, regardless of whether a mutation is recessive or dominant, the presence of the mutated allele leads to the genetic disorder.

Genetic Diseases Examples

There are many examples of genetic disorders. Here we are going to explore two examples.

Polydactyly

Polydactyly is a genetic disorder that frequently runs in families and results in one or more additional fingers that are often small and underdeveloped.

Polydactyly is an autosomal dominant disorder. This means that possessing only a single mutated allele (P) is enough for the disease to occur.

In other words, a heterozygous person (Pp) for the polydactyly allele will develop the disorder.

Furthermore, this person will produce two types of gametes regarding this gene—one with the healthy allele (p) and another with the mutant allele (P). Hence, there will be a 50:50 chance that their children will inherit the mutant allele and thus the condition. We can use a punnet square to elaborate this further:

The removal of the extra finger is the most common treatment for polydactyly. This is usually performed when the baby is one or two years old.

This age is young enough for children not to miss developmental milestones like grabbing for objects yet be able to tolerate anaesthesia and surgery better.

Cystic fibrosis

Cystic fibrosis (CF) is a genetic disorder caused by mutation of the CFTR (cystic fibrosis transconductance regulator) gene that controls the movement of salt and water in and out of the cells. CF causes thick and sticky mucus to build up in the lungs, digestive system, as well as other organs, causing a plethora of challenging complications throughout the body:

  • In the lungs: the healthy CFTR protein is responsible for keeping the airways moisturised and lubricated. When the CFTR protein is not functioning adequately, however, the secretions in the airways become thick and sticky.

    • As a result, people with CF often present with persistent productive coughs, repeated lung infections, and exercise intolerance.

  • In the pancreas and the gut: the sticky secretions can clog up the pancreatic duct and prevent the secretion of pancreatic enzymes into the small intestine. Pancreatic enzymes are essential for the digestion of food, especially proteins.

    • Therefore, people with CF can suffer from malabsorption.

      Furthermore, CF can also result in bowel obstruction in newborns, which often requires surgery.

Various mutations can cause CF. However, the most common one is an autosomal recessive mutation (F508del) that involves deleting a phenylalanine amino acid in the CFTR protein.

For a person to develop CF, they must have inherited the mutant CFTR allele from both parents.

CF is not curable, but it can be managed. Although cystic fibrosis is progressive and requires daily care, people with CF can usually attend school and work. They often have a better quality of life than people with CF had in previous decades. Improvements in screening and treatments mean that people with CF may live into their mid-to-late 30s or 40s, and some are living into their 50s!

People with CF must receive the proper treatments to live longer. These treatments come in many forms, including antibiotics and rehabilitation therapies.

Inherited Disorders Management

Thanks to advances in modern medicine, there are various ways that genetic disorders can be managed.

Screening for Genetic Disorders

Genetic tests can show if a person carries a mutant allele. Having this information can allow for family planning. In addition, diagnosing a genetic condition early allows for prompt medical treatment.

Screening can be done on an embryo or a fetus. Some cells must be harvested from the subject to perform a genetic test. There are various ways that this can be done:

  1. Amniocentesis: this method involves obtaining fetal cells found in the amniotic fluid and can be performed at approximately 15-16 weeks of pregnancy.

    Amniotic fluid is the fluid that the fetus is submerged in.

  2. Chorionic villus sampling: This method is performed slightly earlier, at 10-12 weeks of pregnancy, and involves taking a small sample of cells from the fetal placenta.

  3. IVF (in vitro fertilization): This method can only be performed during IVF and involves collecting some cells from the embryos before implanting them in the mother.

    This would allow for selecting embryos with no genetic disorders, which raises some ethical issues.

Once the fetal or embryonic cells are harvested, they are tested for carrying specific genetic disorders. This gives the parents more options. They may decide to keep the baby despite knowing they will have a genetic disorder. Some others may choose to have an abortion and prevent the birth of a child with a severe genetic disorder.

Treatment of Genetic Disorders

Curing genetic disorders is still at the research stage and has not reached clinical application. Scientists are hoping to be able to use genetic engineering to replace faulty alleles with healthy ones.

Onasemnogene abeparvovec, also known as Zolgensma, is one of the first gene therapy medications that has been successfully used to cure spinal muscular atrophy (SMA). This condition is caused by a mutation in the SMN1 gene, which causes progressive loss of muscle function in children at a young age.

Zolgensma is a one-time medication that works by providing a new copy of the gene, helping to treat this hereditary condition. Zolgensma is one of the most expensive medications covered by NHS at a whopping price of £1.795 million per single dose!

Inherited Disorders - Key takeaways

  • Inherited diseases are disorders that are caused by a mutation in the genome.

  • Mutations are abnormalities and changes in the genome that can occur due to DNA synthesis mistakes or exposure to mutagenic agents.

  • Genetic disorders can be classified as recessive or dominant; monogenic, chromosomal or complex; and sex-linked or autosomal.

  • Polydactyly (caused by an autosomal dominant mutation) and cystic fibrosis (caused by an autosomal recessive mutation) are genetic disorders.

  • Screening for genetic disorders can be carried out by amniocentesis, chorionic villus sampling or during IVF.

Frequently Asked Questions about Inherited Disorders

Inherited diseases are disorders that are caused by a mutation in the genome that can be passed from one generation to the next. 

it is thought that bipolar disorder is linked to genetics since it appears to run in families. However, no single gene associated with bipolar disorder has not been identified yet. 

Cystic fibrosis, sickle cell anaemia, polydactyly, and haemophilia are examples of inherited disorders. 

inherited disorders can be classified into sex-linked and autosomal, with reference to which chromosome contains the gene that causes the condition. 

Inherited diseases are disorders that are caused by a mutation in the genome. Mutations are essentially abnormalities and changes in the genome. They may occur due to a mistake during DNA replication and cell division. Mutations can also occur due to environmental factors such as UV light exposure or mutagenic chemicals. 

Final Inherited Disorders Quiz

Question

What are genetic disorders?

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Answer

Inherited diseases are disorders that are caused by a mutation in the genome that can be passed from one generation to the next.  

Show question

Question

Define mutations

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Answer

Mutations are essentially abnormalities and changes in the genome that can occur due to DNA synthesis mistakes or exposure to mutagenic agents. 

Show question

Question

__________ mutations changes in one or more nucleotides that only affect a single gene. 

Show answer

Answer

Small-scale 

Show question

Question

____________ mutations (also known as ___________ mutations) involve changes in a long segment of DNA, at a chromosomal level. Therefore, these mutations often affect multiple genes. 

Show answer

Answer

Large-scale, chromosomal

Show question

Question

How many dominant mutant alleles are needed to cause disease?

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Answer

Only one mutant allele

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Question

How many recessive mutant alleles are needed to cause disease? 

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Answer

Two recessive mutant alleles

Show question

Question

Define monogenic disorders and provide examples.

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Answer

Monogenic disorders are the result of a mutation in a single gene. Examples of monogenic disorders include sickle cell disease and cystic fibrosis.  

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Question

What are complex disorders?

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Answer

In complex genetic disorders, a combination of gene mutations and other factors, such as diet and chemical exposure, are responsible for the disease. These disorders are also called multifactorial.   

Show question

Question

What is polydactyly?

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Answer

Polydactyly is a genetic disorder that frequently runs in families and results in one or more additional fingers that are often small and underdeveloped. 

Show question

Question

A man is homozygous for the polydactyly allele. What portion of his children will have polydactyly?

Show answer

Answer

All of them

Show question

Question

How is polydactyly treated?

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Answer

The removal of the extra finger is the most common treatment for polydactyly.  

Show question

Question

What is cystic fibrosis?

Show answer

Answer

Cystic fibrosis (CF) is a genetic disorder caused by mutation of the CFTR (cystic fibrosis transconductance regulator) gene that controls the movement of salt and water in and out of the cells. 

Show question

Question

How does cystic fibrosis affect the lungs?

Show answer

Answer

In the lungs, the healthy CFTR protein is responsible for keeping the airways moisturised and lubricated. When the CFTR protein is not functioning adequately, however, the secretions in the airways become thick and sticky. As a result of this, people with CF often present with persistent productive coughs, repeated lung infections, and exercise intolerance. 

Show question

Question

How does cystic fibrosis affect the pancreas?

Show answer

Answer

The sticky secretions can clog up the pancreatic duct and prevent the secretion of pancreatic enzymes into the small intestine. Pancreatic enzymes are essential for the digestion of food, especially proteins, therefore people with CF can suffer from malabsorption 

Show question

Question

What is amniocentesis?

Show answer

Answer

Amniocentesis is a type of genetic screening that involves obtaining fetal cells found in the amniotic fluid and can be performed at approximately 15-16 weeks of pregnancy.

Show question

Question

What is chorionic villus sampling?

Show answer

Answer

Chorionic villus sampling is a type of genetic screening performed at 10-12 weeks of pregnancy and involves taking a small sample of cells from the fetal placenta.

Show question

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