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

The definition of Mendelian Disorders refers 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. This topic is included in the Class 12 chapter Principles of Inheritance and Variation in Biology.

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1. What are Mendelian Disorders?
2. Types of Mendelian Genetic Disorders
3. Examples of Mendelian Disorders

What are Mendelian Disorders?

Mendelian disorders are genetic disorders that are caused by mutations in a single gene. These disorders follow a specific inheritance pattern, autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive, according to the principles of inheritance formulated by Gregor Mendel.

These disorders greatly affect human health in the sense that they have significant impacts on the quality of life of an individual. Understanding their nature is beneficial for diagnosis, genetic counselling, and the development of targeted treatments. The study of these rare conditions has provided key insights into genes and heredity.

The father of modern genetics, Gregor Mendel, showed us the basic rules of heredity through his experiments with pea plants in the 19th century. He demonstrated that traits were passed as distinct units now called genes and described laws of segregation and law of independent assortment. His work was completely overlooked at the time but has become the foundation for modern genetics and our knowledge of hereditary diseases.

Also Read:

• Practice MCQ on Mendelian Genetics
• Principles of Inheritance and Variation
• Genetics
• Molecular Basis of Inheritance

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: This disorder is caused by 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

• Sickle cell anaemia affects haemoglobin in red blood cells, causing them to take on a rigid, sickle-shaped form.

• It causes anaemia and pain, which are 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

• It is a blood disorder that affects haemoglobin 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)

• Phenylketonuria 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

• Haemophilia is 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.

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