If you’re one of the millions of people who have a genetic disease, you know that it can be tough living with it. But thanks to recent advances in genetic testing and therapy, many patients are now able to manage their conditions more effectively. In this blog, we take a look at five genetic diseases and their causes.
What are the causes of genetic disorders?
To understand the causes of genetic disorders’ it’s helpful to learn more about how your genes and DNA work.
DNA is the building block of all living things, providing instructions for how an organism should develop and function. DNA is made up of a long chain of smaller units called nucleotides. Each nucleotide consists of a sugar molecule, a phosphate group, and one of four nitrogen-containing bases: adenine (A), thymine (T), guanine (G), or cytosine (C). These bases pair up in specific ways (A with T, and G with C) to form the “rings” of the DNA ladder. The sequence of bases in DNA determines the sequence of amino acids in proteins, which in turn dictates the structure and function of those proteins.
The DNA inside our cells contains the instructions needed to build and maintain our bodies. It is responsible for everything from the colour of our hair to the size of our feet. Our DNA is organized into long strands called chromosomes. We inherit half of our chromosomes from our mother and half from our father.
Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. One set of 23 comes from the mother, and the other set comes from the father. Every cell in the human body contains a complete set of chromosomes. When a baby is conceived, the father’s sperm cell and the mother’s egg cell come together, each contributing 23 chromosomes. This process is called fertilisation. The resulting fertilized egg cell (or zygote) has 46 chromosomes – half from the mother and half from the father. From this point on, the cells created through cell division will all contain this same combination of maternal and paternal chromosomes.
This means that we inherit a copy of each of our parent’s genes. Genes are the units of inheritance that dictate how our bodies look and function. They are passed down from generation to generation and determine things like eye colour, height, and risk for certain diseases and you can even inherit a gene mutation. Together, our genes make up our genome, which is the complete set of genetic instructions inside our cells. Although we inherit our genomes from our parents, they are not static. They can change over time, which is how we can adapt to different environments and evolve as a species. Just like fingerprints, every person’s genome is unique. Your genome is what makes you, you!
What are inherited traits?
Inherited traits are characteristics that are passed down from parents to their offspring. A phenotype is an individual’s observable traits. These can be physical traits, such as eye colour or hair texture, or behavioural traits, such as the tendency to be shy or outgoing.
Inherited traits are determined by genes, which are units of information that are passed down from generation to generation. Most inherited traits are controlled by a single gene, but some traits involve multiple genes. For example, height is determined by both genes and environment. People inherit two copies of every gene, one from each parent. If both copies of a gene are the same, then the person is said to be homozygous for that gene. If the two copies are different, then the person is heterozygous.
Inherited traits can also be influenced by the environment, even if they are determined by genes. For example, people who are born with the genetic potential to be tall may not reach their full height if they don’t have enough to eat during childhood. Therefore, it’s important to remember that inherited traits are not always fixed and immutable. They can be influenced by both genes and the environment.
What are the different types of inheritance?
One of the key concepts in genetics is inheritance or the passing down of traits from parent to child. There are two main types of inheritance: autosomal dominant and autosomal recessive.
Autosomal dominant inheritance occurs when a gene mutation is present on an autosome (a chromosome that is not a sex chromosome), and only one copy of the mutated gene is necessary for the trait to be expressed. Only one parent needs the mutated gene to pass it on.
In contrast, autosomal recessive inheritance occurs when a gene mutation is present on an autosome, but two copies of the mutated gene are necessary for the trait to be expressed. Both parents need to carry the mutated gene to pass it on.
Knowing the difference between these two types of inheritance is important for understanding how genetic disorders are passed down within families.
What are mutations?
Mutations are changes in the DNA sequence that can cause genetic mutations. Mutations happen when there is a mistake in copying DNA during cell division. e.g. one base in a DNA sequence might be replaced with another. This can happen naturally or randomly but is more likely to happen if there is damage to the DNA as a result of environmental factors such as:
- UV radiation or
- Exposure to certain chemicals
- Radiation exposure
What are the different types of genetic disorders?
There are three main types of genetic disorders:
- Single-gene: Single-gene disorders are caused by a mutation in one gene. Sickle cell disease
- Chromosome: Chromosome disorders occur when there is a change in the number or structure of the chromosomes. Down Syndrome
- Multifactorial: Multifactorial disorders are caused by a combination of environmental and genetic factors. An example of a multifactorial disorder is heart disease, which can be affected by diet, lifestyle, and family history. Alzheimer’s Disease is another multifactorial disorder
Five genetic diseases
Down syndrome is a common genetic disorder that occurs when a person has an extra copy of chromosome 21. This extra chromosome can cause physical and intellectual problems. People with Down syndrome often have distinct facial features, such as a flattened nose and an upward slant to the eyes. They also tend to be shorter than average and have reduced muscle tone. Many people with Down syndrome also have some degree of intellectual disability.
There are three different types of Down syndrome, which are caused by different mechanisms. The most common type, called trisomy 21, occurs when a person has three copies of chromosome 21 instead of two. This type accounts for about 95% of all cases of Down syndrome and occurs randomly and isn’t inherited. The other two types are much less common and are caused by chromosomal abnormalities in the egg or sperm cells. The risk of having a child with Down syndrome increases with the age of the mother, with women over the age of 35 being at greater risk. The exact reason is unknown but may be related to the age of the mother’s egg cells.
People with Down syndrome typically have a shorter life expectancy than the general population, but this has been increasing in recent years as medical care improves. Individuals with Down Syndrome are also at an increased risk for certain health problems, such as heart defects and respiratory problems. With early intervention and proper support, people with Down syndrome can lead happy and fulfilling lives.
Cystic fibrosis (CF) is a chronic, progressive disease that affects the lungs and digestive system. It is inherited when both parents contain the cystic fibrosis gene.
The disease is caused by a mutation in the gene that encodes the protein cystic fibrosis transmembrane conductance regulator (CFTR). This protein is found in cells that line the airways and promote the movement of salt and water across cell membranes. In people with CF, the mutated CFTR protein does not function properly, leading to the buildup of thick mucus in the lungs and digestive tract. This mucus can clog the airways and trap bacteria, leading to lung infections and inflammation. The constant cycle of infection and inflammation can damage the lungs over time and lead to respiratory failure.
Cystic fibrosis is autosomal recessive, which means that a person must inherit two copies of the mutated CFTR gene in order to develop the disease. A person with only one copy of the mutated gene is a carrier but does not usually experience any symptoms. When two carriers have a child, there is a 25% chance that the child will inherit two copies of the mutated gene and will develop cystic fibrosis. There is also a 50% chance that the child will inherit one copy of the mutated gene and will be a carrier, and a 25% chance that the child will inherit two normal copies of the gene and will not be affected by the disorder.
People with CF often experience persistent cough, shortness of breath, wheezing, and chest pain. They may also have difficulty gaining weight, frequent stomach pain, and greasy stools. There is no cure for CF, but treatments are available to help manage the symptoms and slow down the progression of the disease. With early diagnosis and aggressive treatment, people with CF are now living longer and healthier lives than ever before.
Sickle cell disease (sickle cell anaemia)
Sickle cell disease is a genetic disorder that affects the production of haemoglobin, the protein in red blood cells that carries oxygen. Sickle cell disease is inherited when a person inherits two abnormal haemoglobin genes, one from each parent. People with only one abnormal gene do not have the disease, but they are carriers of the disease and can pass it on to their children.
A sickle cell carrier is someone who has inherited the sickle cell gene from one parent. Although they do not have sickle cell disease, they can pass the gene on to their children. Carriers typically have no symptoms, but they can be at risk for complications if they are undergoing surgery under general anaesthetic or during regular intensive physical activity.
People with sickle cell disease have abnormal haemoglobin, which causes their red blood cells to become crescent-shaped and sticky. These abnormal red blood cells can block blood flow and cause pain, infection, and organ damage. There is no cure for sickle cell disease, but treatment can help to relieve symptoms and prevent complications. Although sickle cell disease is significantly higher among people of African or Caribbean descent, it can affect anyone whose parents carry the sickle cell gene.
Turner syndrome is a disorder that affects only females. It occurs when one of the two X chromosomes is missing or incomplete. In some cases, the missing chromosome is completely lost. In other cases, deletion of part of the chromosome occurs or is translocated to another location in the genome. Turner syndrome is caused by a random error that occurs during cell division in early embryonic development. The disorder is not inherited and affects only females.
It is characterised by physical abnormalities, including short stature, a webbed neck, and lymphedema (swelling of the hands and feet). The most common symptom of Turner syndrome is short stature, which is often apparent by age 8-14. Most girls with Turner syndrome are diagnosed before they reach puberty, although some may not be diagnosed until they reach adulthood. Most affected individuals also have difficulty functioning ovaries, which can lead to infertility and they often have heart defects or kidney problems.
Treatment typically involves hormone therapy and fertility treatments. The long-term outlook depends on the severity of symptoms and whether complications develop. There is no cure for Turner syndrome, but early diagnosis and treatment can help girls manage their symptoms and live healthy, fulfilling lives.
Huntington’s disease (HD) is an autosomal dominant inherited disease that affects the brain. It usually develops in adulthood, and it causes a gradual decline in mental abilities and physical function. The most common symptom of Huntington’s disease is involuntary movements of the face and body (known as chorea). Other symptoms can include depression, anxiety, irritability, and difficulties with speech and swallowing. HD is caused by a mutation in the HTT gene. This gene is found on chromosome 4, and it provides instructions for making a protein called huntingtin. This protein is found throughout the body, but its exact function is not known. The mutated huntingtin protein damages nerve cells in the brain, which leads to the symptoms of Huntington’s disease. HD is a progressive condition, which means that it gradually gets worse over time. There is currently no cure for HD, but treatments are available to help manage the symptoms.
Other inherited disorders
- Duchenne muscular dystrophy is muscular dystrophy caused by mutations in the gene for dystrophin, which leads to progressive muscle weakness
- Cri du chat syndrome is a rare disease where a part of chromosome 5 is missing or deleted
- Fragile X syndrome is an X-linked single-gene disorder
- Neurofibromatosis is caused by a faulty NF1 gene that leads to tumours developing in the nervous system
- Klinefelter syndrome is where males are born with an extra X chromosome
- Tay-Sachs disease is a genetic condition that results in the progressive destruction of the nervous system from a defect found in chromosome 15
- Thalassemia is caused by errors in the genes for haemoglobin similar to sickle cell disease
Genetic diseases are passed down from parents to their children. Sometimes, a genetic disease can be identified before birth through prenatal testing. Other times, the disease may not be discovered until symptoms appear. There is no right way to deal with a genetic disease – it depends on the individual and the type of disease involved. Some people choose to receive treatment, while others decide to live with the condition. In some cases, gene therapy or other treatments may become available in the future. If you have a family history of genetic diseases, speak to your healthcare professional about getting tested for those conditions. Early diagnosis can help you make informed decisions about your health care.
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