Mendelian Disorders


Gene is a fundamental physical and functional unit of heredity. It is composed of DNA that carries genetic information, which translates proteins and hereditary characteristics from parents to their offspring. They can undergo mutations and produce faulty proteins, therefore, it leads to genetic disorders. The inheritance patterns in pea plants studied by Mendel are a strong foundation to understand singlegene disorders in humans. There are many genetic disorders developed from birth and few disorders can be acquired because of the changes in genes. For example, cancer can be caused by congenital genes or by mutation due to environmental factors.

What are Mendelian Disorders?

Mendelian disorders are a type of genetic disorder that occurs due to the alteration in a single gene or because of a defect in the genome. It can be observed from birth and it can be found based on family history.

Types of Mendelian Genetic Disorders

There are various types of Mendelian Disorders according to Mendel’s Laws of Inheritance and are identified from the pedigree chart.

Types of Mendelian disorders are as follows −

  • Autosomal recessive

  • Autosomal dominant

  • X-linked recessive

  • X-linked dominant

  • Mitochondrial

Examples of Mendelian Disorders

Sickle-cell anemia, hemophilia, phenylketonuria, thalassemia, cystic fibrosis, color blindness, skeletal dysplasia, achondroplasia, and muscular dystrophy are some examples of Mendelian disorders.


It is an inherited bleeding disorder that is the condition of a moderate blood clotting process. It is characterized by inadequate levels of clotting factors VIII or IX. Males can get easily affected by this disorder because the affected gene is present on the X chromosomes.


  • Nose bleeds

  • Pain and swelling in joints

  • Large bruises

  • Neck pain

  • Frequent vomiting

  • Extreme post-surgical bleeding

  • Headache

  • Dizziness

  • Bloody urine or stools


  • Hemophilia A is the most common type and is also called classical hemophilia. It is caused by the lack of plasma protein factor VIII. A person with hemophilia A may experience moderate to severe symptoms

  • Hemophilia B is a less common type and is also called Christmas disease. It is caused by the deficiency of plasma protein factor IX. A person with hemophilia B may experience mild, moderate, or severe symptoms.

Sickle-cell Anemia

It is one of the inherited disorders that affect the shape of red blood cells and it is caused by defective genes. RBCs can carry oxygen to all parts of the body and are sickle or crescent moon-shaped cells in the condition of sickle cell anemia. Therefore, the cells become hard and adhere and they move slowly or block the flow of the blood.


  • Anemia

  • Swollen hands and feet

  • Extreme pain in the abdomen and chest

  • Vision problems

  • Persistent infections

  • Delayed growth

  • Severe pain in joints


  • Stroke

  • Pulmonary hypertension

  • Acute chest syndrome

  • Gallstones

  • Organ damage

  • Blindness

  • Pregnancy complications

Phenylketonuria (PKU)

It is a rare genetic condition and is also called phenylalanine hydroxylase deficiency. The deficiency of the enzyme weakens the ability of the body to metabolize the essential amino acid phenylalanine. The deposition of phenylalanine in the body impacts cognitive functions and creates severe intellectual disability.


  • Tremors

  • Learning disabilities

  • Hyperactivity

  • Seizures

  • Eczema

  • Mental retardation

  • Epilepsy

  • A musty odor in their skin and hair

  • Slow growth

Special diets

A child with PKU requires a special milk formula and needs a diet containing low levels of phenylalanine. A special diet is essential for the child to prevent the destruction of the brain even when it is growing. The diet should be recommended throughout life to enhance their concentration, performance in school, and capacity to think.


It is a genetically acquired blood disorder and affects the ability of the body to produce hemoglobin and red blood cells. It can cause mild or moderate anemia and becomes life-threatening.


  • Drowsiness

  • Jaundice

  • Shortness of breath

  • Fast heartbeat

  • Dizziness

  • Susceptible to infection

  • Delayed growth

  • Pale skin

  • Heart problem


Hemoglobin can be usually made by chains of four alpha-globin proteins and two beta-globin proteins.

  • Alpha thalassemia occurs when two or more alpha-globin genes are absent or defective.

  • Beta thalassemia is the most common type of thalassemia that occurs when one or both beta-globin genes are absent or defective. A person with betathalassemia has low levels of hemoglobin which leads to a deficiency of oxygen in several parts of the body.

Cystic Fibrosis

It is also a genetically acquired disease that damages the lungs, pancreas, digestive tract, and other organs by developing thick and sticky mucus in organs.

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  • Jaundice

  • Lung infections

  • Diarrhea

  • Coughing, wheezing, and shortness of breath

  • Breathing difficulties

  • Poor growth

  • Extreme salty sweat

  • Fatigue

  • Nasal polyps


  • Malnutrition

  • Diabetes

  • Arthritis and osteoporosis

  • Colorectal cancer

  • Infertility

  • Chronic respiratory failure

  • Liver diseases

  • Bowel problems

  • Pneumonia


Gene is the basic unit of heredity and it transfers genetic information from one generation to another generation. The genetic disorders may or may not be acquired from the parents. The Mendelian disorder is a single-gene disorder caused by specific mutations or abnormalities. There are several Mendelian disorders in humans and they can be classified based on Mendel’s Laws of Inheritance. It includes sickle cell anemia, phenylketonuria, cystic fibrosis, hemophilia, albinism, thalassemia, muscular dystrophy, and some other genetic disorders. Sickle cell anemia is one of the inherited disorders that affect the shape of red blood cells. Phenylketonuria is developed by the deficiency of the enzyme phenylalanine hydroxylase. Thalassemia is a blood disorder that is characterized by the inability to produce hemoglobin and RBC.


Q1. How does chromosomal disorder differ from Mendelian disorder?

Ans. The chromosomal disorder is caused because of the abnormal arrangement or lack or addition of chromosomes. Turner’s syndrome is an example of this disorder. Mendelian disorder is caused due to changes or mutations in a single gene. Hemophilia is a suitable example of a Mendelian disorder.

Q2. Why is sickle cell anemia a Mendelian disorder?

Ans. Sickle cell anemia is an autosomal recessive disorder. It is controlled by a locus in an inheritance pattern and mutation in one gene causes the Mendelian disorder.

Q3. How is thalassemia cured?

Ans. Thalassemia is cured by stem cell therapy. This is the only treatment involved but there is a higher risk. Frequent blood transfusions are required for the patient.

Q4. Why is a special diet needed for a child with phenylketonuria?

Ans. A special diet is essential for a child with PKU to prevent the destruction of the brain even when it is growing. It is because of the condition PKU that the enzyme deficiency leads to the weakening of the body and cannot metabolize the amino acid phenylalanine. Therefore, special milk formulas and phenylalanine-free foods are required for children.

Q5. Why do men easily get hemophilia?

Ans. Men have only one copy of genes on the X chromosome and they inherit an affected X chromosome that has a mutation in the clotting factor VIII or IX gene. Therefore, men can easily be affected by hemophilia.

Updated on: 09-Jan-2023


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