Mendelian Disorders: Definition, Types, Development, Examples, Facts

Definition Of Mendelian Disorders

Mendelian disorders refer to those genetic disorders caused due to mutations in a single gene. The Mendelian Disorders are caused due to the inheritance of genes by laws formulated based on the work of Gregor Mendel. These disorders can be predictable, autosomal dominant, autosomal recessive, or linked to the X chromosome.

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This Story also Contains

1. Definition Of Mendelian Disorders
2. What Are Mendelian Disorders?
3. Types Of Mendelian Genetic Disorders
4. Examples Of Mendelian Disorders
5. The Video Recommended On Mendelian Disorders:

What Are Mendelian Disorders?

Mendelian disorders represent a group of genetic disorders arising due to changes or mutations in a single gene. These occur according to specific patterns of inheritance: autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive, which are based on Gregor Mendel's laws of inheritance.

The Mendelian disorders have a bearing on the significant issues that pertain to human health for the simple reason that they represent several inherited conditions that can significantly affect an individual's quality of life. Their nature must be understood to provide diagnosis, genetic counselling, and to develop directed therapies. The study of these relatively rare disorders, in comparison with multifactorial diseases, has provided insights into even the very basics of mechanisms that relate to genes and heredity.

Gregor Mendel has been called the father of modern genetics. The focus of Mendel's work, which took place in the middle of the nineteenth century with garden pea experiments, was on the basic principles of heredity. He stated that particular characteristics are passed on as discrete units, now called genes, and formulated the laws of segregation and independent assortment. Mendel's work, overlooked initially, formed the centrepiece for modern genetics and today's knowledge base about hereditary diseases.

Types Of Mendelian Genetic Disorders

Mendelian genetic disorders are classified as:

Autosomal Dominant Disorders

These disorders only need one copy of the mutated gene for the disorder's expression. There is a 50% probability that the disorder will be passed on to progeny by the patient.

Examples:

• Huntington's Disease: Neurodegenerative disorder causing progressive motor deterioration, cognitive decline, and psychiatric symptoms.

• Marfan Syndrome: An inherited connective tissue disorder that affects the heart, eyes, blood vessels, and skeleton, leading to cardiovascular complications.

Autosomal Recessive Disorders

It is present only in those who have inherited the mutated gene in duplicate from the two parents. Carriers, who have one copy of the mutated gene are mostly asymptomatic.

Examples:

• Cystic Fibrosis: Autosomal recessive, wherein a person experiences chronic complications of the lungs and digestive system.

• Sickle Cell Anemia: An autosomal genetic disorder whereby the red blood cells are in a crescent shape, causing anaemia, pain, and many other serious complications.

X-Linked Dominant Disorders

These are due to genes which have a mutation on the X chromosome. A single copy of the gene can mean that both males, who have one copy of the X chromosome, and females, who have two copies of the X chromosome, have the disorder.

Examples:

• Rett Syndrome: A neurodevelopmental disorder almost exclusively affecting girls, where there is normal early growth followed by the loss of motor skills and speech.

X-Linked Recessive Disorders

These disorders are also due to a gene mutation on the X chromosome. Males are more involved because of having only one X chromosome. Females, being equipped with two X chromosomes, are generally carriers unless they inherit two copies of the mutated gene.

Examples:

• Hemophilia: Disorder caused due to an inability of blood to clot, resulting in excessive bleeding.

• Duchenne Muscular Dystrophy: Progressive degeneration and weakness of muscles.

Examples Of Mendelian Disorders

The examples are described below:

Sickle Cell Anaemia

• Affects hemoglobin in red blood cells, causing them to take on a rigid, sickle-shaped form.

• Causes anaemia, pain—termed sickle cell crises and possible organ damage.

• Patients have increased susceptibility to infection due to spleen damage.

• Those with complications such as stroke, acute chest syndrome, and pulmonary hypertension.

• Among the modalities of management include pain relief, blood transfusions, and drugs like hydroxyurea.

Muscular Dystrophy

• These are a group of genetic disorders that result in muscle weakness and degeneration.

• The type that is most common and most severe is Duchenne muscular dystrophy.

• Duchenne muscular dystrophy (DMD), occurs almost exclusively in boys. The first symptoms usually appear at an early age.

• Characterised by progressive muscle wasting, leading to loss of ambulation

• May result in respiratory and cardiac complications

• Treatment focuses on physical therapy, steroids, and supportive care

Cystic Fibrosis

• Causes thick, sticky mucus to build up in the lungs and digestive tract

• This leads to severe respiratory issues, frequent lung infections, and difficulty breathing.

• Causes pancreatic enzyme insufficiency due to the inability to activate these enzymes, leading to malnutrition and poor growth.

• It can also lead to the development of diabetes mellitus, liver disease, and male infertility.

• Some interventions used for treatment involve airway clearance techniques with the aid of enzyme supplements and medications such as CFTR modulators.

Thalassemia

• Is a blood disorder that affects hemoglobin production.

• The causes anaemia and hence fatigue, weakness, and pale or yellowish skin.

• The severe forms like beta-thalassemia major are transfusion-dependent.

• The complications can also include iron overload, bone deforming, and spleen enlargement.

• Treatments range from blood transfusions and iron chelating to even bone marrow transplants in some cases.

Phenylketonuria (PKU)

• This is a metabolic disorder whereby the body lacks one enzyme that will break down the amino acid phenylalanine.

• If not treated, the phenylalanine continues to build up, causing damage to the brain and hence leading to intellectual disability and developmental delay.

• The symptoms are musty odour, seizures, and behavioural problems.

• Diagnosis by newborn screening.

• Treatment includes a strict diet with low phenylalanine levels in food and special medical formulas.

Colour Blindness

• An inability of a person to see certain colours or differentiate between them, mostly red and green.

• The causes of colour blindness are usually mutations in genes for colour vision, mostly on the X chromosome.

• More common in males due to X-linked inheritance.

• It does not usually cause much disability but affects daily activities.

• Coping strategies and tools available e.g. colour corrective lenses. No cure is available.

Skeletal Dysplasia

• A collection of disorders that affect the growth of bone, resulting in stunted average height with a host of skeletal defects or anomalies. Amongst others, they include achondroplasia and osteogenesis imperfecta.

• Symptoms include disproportionate limb length, joint problems, and spinal problems. It can also result in respiratory problems and many more.

• Complications, such as hearing loss and dental problems. Cured by orthopaedic interventions, physical therapy, and surgical means in some cases.

Haemophilia

• A bleeding disorder whereby the blood does not clot and suffers from excessive bleeding from minor wounds.

• The common forms are haemophilia A and haemophilia B, due to a deficiency of clotting factor VIII and clotting factor IX, respectively. This can cause spontaneous bleeding into joints and muscles with pain and disability.

• Severe cases may be associated with life-threatening haemorrhages.

• Treated by regular clotting factor replacement therapy and preventive measures.

Conclusion

Mendelian disorders, based upon single-gene mutation, are of prime significance and cannot be dispensed with in gaining insights into genetic inheritance and the impact it has on human health. Awareness and education about such disorders can help in the diagnosis and management of affected patients and also in the sphere of genetic counselling. More research in genetics soon may bring out better treatments and one day even find a cure for these inherited conditions that enhance the quality of life of affected individuals.

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