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“Write an essay describing a genetic disorder with a defined inheritance pattern.
Include details of the pattern of inheritance, the biology of the condition, the disease
profile in patients and existing and emerging treatments”
The genetic disorder I have chosen to write about is Duchenne muscular dystrophy, which is a condition that cause a reduced lifespan through the progressive weakening of the muscles. In this essay I aim to describe how this disease is inherited, how it affects individuals and the current and potential treatments for Duchenne muscular dystrophy.
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This genetic condition is a recessive disorder but as it is X-linked, males only require a single copy and therefore affects more males than females, as females require two copies of the affected chromosome, which is unlikely to occur as most males do not live to reproduce. (1) This means that there will be carrier females and affected males will either inherit the disease from their mother or from a random mutation. Therefore, there is a likely chance that the condition will skip generations, or a milder phenotype might be expressed in carriers. It affects one in every three thousand five hundred males. (2) As it is congenital, there is no way to prevent the disorder in patients who had random mutations, but genetic testing of mothers is possible, and this will help parents make decisions about their future families.
The gene affected is the largest in the human genome and is on the p-arm of the X chromosome and it codes for dystrophin, an anchoring protein for muscles. (1) Deletions and insertions are common mutations, and these are likely to cause a frameshift which will change the whole protein or cause the protein to not be expressed. (2) Dystrophin links the cytoskeleton to the extracellular matrix via the dystrophin associated protein complex but also has a cell-signalling role which is also linked to the pathogenesis. (2) Loss of function of dystrophin because of genetic mutations leads to instability and membrane leakage. (2)
Symptoms usually occur in childhood, with patients characteristically having problems in physical tasks, such as climbing the stairs, walking without falling over and usually struggle to keep up with their friends. (1) A typical sign as well is Gowers’ manoeuvre, which is when the patient “walks up their own legs” to get up and is an indication of weakness in extensors at the joints in your legs. (3) A family history of the disease will also make diagnosis easier as it is a genetic disorder but if the history is unknown or there is no history, a test for creatine kinase can be done, as this enzyme is released in damaged muscle. (1)
As the affected patient ages, their symptoms will get progressively worse. Duchenne muscular dystrophy starts affecting all the muscles in the body, including muscles in the heart and lungs, so fatigue and severe difficulty in movement and breathing regularly occurs, usually in late childhood. (4) Patients are only able to walk independently up until around twelve years of age and afterwards will require a wheelchair. (2) As the muscle destruction caused by Duchenne muscular dystrophy gets worse, patients will have respiratory and gastrointestinal problems as well. (5) Other symptoms that some patients may have are scoliosis, muscle contractures that can prevent a joint from moving and sometimes, learning difficulties. (1) Abnormal electrocardiograms are common at age eighteen and cardiac or respiratory failure are usually the cause of deaths in patients. (2)
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There is no current cure for Duchenne muscular dystrophy, so current treatments are focused on alleviating symptoms. Physical therapy is required for patients to strengthen muscles and for patients to develop a full range of motion, which is needed to counteract the contractures. (1) The NHS uses corticosteroids which can improve the strength of the patient’s muscles for a short period of time, usually not longer than two years. (6) For respiratory problems, patients may require ventilation to compensate for the weakness of the muscles and to treat cardiac problems, beta-blockers can be used. (1)
An emerging treatment for Duchenne muscular dystrophy is the use of viruses to establish proper function of muscle cells by delivering the gene for dystrophin. (7) A problem with this treatment is that it would require injections to muscles all over the body and that gene replacement will be difficult due to the size of the dystrophin gene, as the potential vectors used may not be able to carry this gene. (7)
Another therapy that is being researched for Duchenne muscular dystrophy, is the promotion of exon skipping during RNA splicing. (8) This therapy could be successful as it would negate frameshift mutations and produce a partially functional version of dystrophin. (8) A problem with exon skipping is that it would not cure the genetic disorder but would help to restore some integrity to the muscles of the patient and make them stronger.
Stem cell therapy is another potential treatment for Duchenne muscular dystrophy. In this treatment, stem cells are taken from the patient, edited in vitro to restore the dystrophin expression and then they are implanted back into the patient or stem cells are taken from a person with functional dystrophin and donated. (9) A problem with this is that the donor stem cells can be rejected by the patient, which means that there is little possibility that stem cells could be delivered directly into the patient’s circulation.
To conclude, Duchenne muscular dystrophy is a severe, genetic disorder of the dystrophin gene that is present in affected males from an early age and causes early mortality, commonly by respiratory or cardiac failure. Females are not usually affected, as they need to be homozygous for the condition because it is X-linked recessive, but they may be manifesting carriers.
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