Human Genetic Disorders: Definition, Types, Development, Examples, Facts

Human genetic disorders are medical conditions arising caused by abnormalities in the DNA of an individual. These may result from changes in a single gene, multiple genes, or entire genes. Such disorders may or may not be inherited from parents and arise due to spontaneous mutations. Advances in genetics have made it easier to understand the molecular basis of these disorders.

This Story also Contains

1. Genetic Disorders
2. Types of Genetic Disorders
3. Causes of Chromosomal Disorders
4. Genetic Counselling
5. List of Genetic Disorders
6. The Video Recommended on Genetic Disorders
7. MCQs on Human Genetic Disorders

Many human genetic disorders are identified and traced using pedigree analysis. This helps in genetic counselling and in determining patterns of inheritance such as autosomal dominant, autosomal recessive, or X-linked. Understanding these patterns is essential in assessing risks, planning treatment, and further research in genetics

Genetic Disorders

Genetic disorders are caused by changes or abnormalities in a person’s DNA. These disorders can be inherited from parents or occur as new mutations. Such disorders may involve abnormalities at the single gene, several genes, or even chromosome levels. They have effects on diverse body functions and development.

The study of genetic disorders has helped scientists and doctors to understand the root of various diseases. This knowledge is important for better diagnosis, treatment, and prevention strategies. It also provides insights into how genes work and how they influence human development, contributing to medical research and public health.

Genetic inheritance is the process by which traits are passed from parents to their offspring. Disorders can be inherited in different patterns such as autosomal dominant, autosomal recessive, X-linked, and mitochondrial inheritance. Each pattern explains how a disorder may appear in a family and how likely it is to be passed on to successive generations.

Types of Genetic Disorders

Genetic disorders can be classified based on nature and location of the genetic changes involved. These disorders may be inherited or arise spontaneously due to different types of mutations in genetic material. The types of genetic disorders are described below:

Single Gene Disorders

Single gene disorders are caused by mutations of a single gene. These conditions can follow different patterns of inheritance, either autosomal dominant, autosomal recessive, or even X-linked. The effects can range from mild to severe, depending on the gene involved and how it functions in the body.

Examples:

• Cystic Fibrosis: It is caused by a mutation in the CFTR gene. This disorder affects the lungs and human digestive system, leading to accumulation of thick mucus and respiratory problems

• Sickle Cell Anemia: A mutation in the HBB gene results in the sickle-shaped erythrocytes. It leads to pain, anemia, and some resistance to malaria.

• Hemophilia: A blood clotting disorder caused by mutations in either the F8 or F9 genes. It disrupts the production of clotting factors, leading to prolonged bleeding from the injured site

Inheritance Patterns

Genetic disorders are inherited in specific ways based on how traits are passed from parents to offspring. These patterns depend on whether the gene is located on an autosome or a sex chromosome, and if one or two copies of the gene are needed to cause the disorder.

• Autosomal Dominant: One copy of the mutated gene is enough for a disorder to be expressed. The affected person usually has one affected parent. For example, Huntington's disease.

• Autosomal Recessive: Both copies of the gene must be mutated for a disorder to express. Carriers (with one mutated gene) usually show no symptoms. For example, cystic fibrosis.

• X-linked: The mutated gene is located on the X chromosome. Since males have only one X chromosome, the disorder is more commonly seen in males than females. For example, haemophilia will have more serious effects in males.

• Y-linked: The mutation is present on the Y chromosome and it is passed only from father to son. These disorders are rare and only affect males. For example, hypertrichosis.

Chromosomal Disorders

Chromosomal disorders occur when there is a change in the number or structure of chromosomes. These changes can happen during gamete formation. These disorders can lead to physical, developmental, or reproductive issues. They can be detected using genetic testing methods like karyotyping.

Examples:

• Down Syndrome: A condition caused by an extra copy of chromosome 21 (Trisomy 21), resulting in intellectual disability and physical abnormalities.

• Turner Syndrome: A condition wherein females have only one X chromosome (XO), which leads to problems in development and reproductive functions.

• Klinefelter Syndrome: A disorder found in males who have an extra X chromosome. The presence of the extra X chromosome makes them XXY, which is associated with reduced levels of testosterone and hence infertility.

Causes of Chromosomal Disorders

Chromosomal abnormalities result from an error in cell division, involving either nondisjunction or translocation events. These abnormalities could further lead to physical and developmental abnormalities whose type depends on the specific chromosomes involved.

Multifactorial Disorders

Multifactorial disorders are caused by combined effects of genetic and environmental factors. These conditions do not follow Mendelian genetics. They involve multiple genes. The risk of these disorders increases when both genetic and environmental factors are present. Indeed, the nature of these interactions is of crucial concern for prevention and cure.

Examples:

• Heart Disease: Arises from interaction between genetic predisposition and behavioural factors such as intake of nutrients, smoking, and exercise.

• Diabetes: Caused by interaction between genetic susceptibility and environmental factors such as obesity and lack of exercise.

• Cleft Palate: Can result from genes and environmental exposures during pregnancy.

Mitochondrial Disorders

Mitochondrial disorders are caused by mutations in mitochondrial DNA (mtDNA). It is found in the semi autonomous organelle, mitochondria. The mt DNA is inherited only from the mother, making these disorders strictly maternally inherited. They mainly affect tissues and organs with high energy demand, such as the brain and muscles. It is because mitochondria are vital for energy production in cells

Examples:

• Leber's Hereditary Optic Neuropathy: Causes sudden loss of vision due to degeneration in the optic nerve.

• MELAS Syndrome: Stands for Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes, affecting multiple systems.

Genetic Counselling

Genetic counselling is a process that provides individuals and families information, advice, and support about genetic disorders. A genetic counselor helps assess the risk of disorders,of inherited, implications of genetic testing, and gives a patient options for the management and prevention of genetic disorders. This is helpful for people with a family history of genetic disorders, coupling intending to start a family, or a person who has been diagnosed with a genetic condition.

Genetic counselling is very important to help people to make informed decisions about their health and the well-being of their families. It involves collecting detailed family history, evaluating genetic risks, discussing possible genetic tests, and reviewing test results. It also offers emotional support, education, and proper resources and support groups to such families. Overall, ensure that the patients and families are truly informed and empowered to cope and live with genetic health problems.

List of Genetic Disorders

Genetic disorders vary in their causes, symptoms, and pattern of inheritance. They affect physical, developmental, or metabolic functions in a human body. It is to identify and study these disorders for better medical care as well as awareness among people. The following is the list of some genetic disorders:

• Cystic Fibrosis

• Sickle Cell Anemia

• Haemophilia

• Down Syndrome

• Turner Syndrome

• Klinefelter Syndrome

• Huntington's Disease

• Tay-Sachs Disease

• Phenylketonuria, PKU

• Duchenne Muscular Dystrophy

• Marfan Syndrome

• Fragile X Syndrome

• Leber's Hereditary Optic Neuropathy

• MELAS Syndrome

• Cleft Palate

The Video Recommended on Genetic Disorders

MCQs on Human Genetic Disorders

Q1. Which of the following is not a Mendelian disorder?

1. Alzheimer's disease

2. Huntington's disease

3. Cystic fibrosis

4. Sickle-cell anaemia

Correct Answer: 1) Alzheimer's disease

Explanation:

Alzheimer's disease is not considered a Mendelian disorder as it is not caused by a single gene mutation but is rather influenced by multiple genes and environmental factors. In contrast, the other options listed are Mendelian disorders caused by mutations in a single gene.

Hence, the correct answer is option 1) Alzheimer's disease.

Q2. Sickle cell anemia is ____ disease.

1. Autosomal recessive

2. Autosomal dominant

3. Pathogenic

4. Mitochondrial Inheritance

Correct Answer: 1) Autosomal recessive

Explanation:

The illness known as sickle cell anaemia is autosomal recessive. Accordingly, a person cannot get sickle cell disease unless they inherit two copies of the gene, one from each parent. A person is referred to as a carrier (heterozygous) if they inherit only one copy of the sickle cell gene. They usually do not exhibit signs of the disease, but they can pass the gene on to their children.

Hence, the correct answer is option 1) Autosomal recessive.

Q3. Phenylketonuria, the genetic disorder causing an inborn error of metabolism is an example of

1. Autosomal dominant

2. Autosomal recessive

3. X-linked dominant

4. X-linked recessive

Correct Answer: 2) Autosomal recessive

Explanation:

Phenylketonuria - Phenylketonuria (PKU) is a rare genetic disorder caused by a mutation in the gene that encodes the enzyme phenylalanine hydroxylase (PAH). This enzyme is responsible for converting the amino acid phenylalanine into tyrosine. A deficiency leads to a toxic buildup of phenylalanine in the body.

• It is an inborn error of metabolism, which is inherited as an autosomal recessive trait.

• It is a rare disease in which the individual is born without the ability to properly break down an amino acid called phenylalanine.

Hence, the correct answer is option 2) Autosomal recessive.

Also Read:

• MCQs on Human Genetic Disorders

• Mendelian Genetics

• Molecular Basis of Mutation