It refers to any disease caused by an anomaly in the genome of an individual, the complete genetic makeup of the person.
The anomaly can vary from lowercase to significant, from a discrete mutation in a single base in the DNA of a single gene to a severe chromosomal abnormality involving the addition or subtraction of a chromosome or set of complete chromosomes.
Some genetic disorders are inherited from the parents, while other genetic diseases are caused by changes or mutations acquired in a gene or group of pre-existing genes.
Mutations can occur randomly or due to some environmental exposure.
What are the different types of inheritance?
There are several different types of genetic inheritance, including the following four modes:
Individual genetic inheritance
Unique genetic inheritance is also called Mendelian or monogenetic inheritance. This type of inheritance is caused by changes or mutations that occur in the DNA sequence of a single gene.
There are more than 6,000 known disorders of a single gene, which occur in approximately 1 in 200 births. These disorders are known as monogenetic disorders (single-gene disorders).
Some examples of monogenetic disorders include:
Disorders of a single gene are inherited in recognizable patterns: autosomal dominant, autosomal recessive, and linked to X.
It is also called complex or polygenic inheritance. Multifactorial inheritance disorders are caused by environmental factors and mutations in multiple genes.
For example, different genes have been found that influence susceptibility to breast cancer on chromosomes 6, 11, 13, 14, 15, 17, and 22. Some common chronic diseases are multifactorial disorders.
Examples of multifactorial inheritance include:
- Heart disease.
- High blood pressure
- Alzheimer’s disease.
Multifactorial inheritance is also associated with hereditary traits, such as fingerprint patterns, height, eye color, and skin color.
Chromosomes, distinct structures formed by DNA and protein, are found in the nucleus of each cell.
Because chromosomes are the carriers of genetic material, anomalies in the number or structure of chromosomes can cause diseases. Abnormalities in chromosomes typically occur due to a problem with cell division.
For example, Down syndrome or trisomy 21 is a common disorder when a person has three copies of chromosome 21. There are many other chromosomal abnormalities, including:
- Turner’s syndrome (45, X0).
- Klinefelter syndrome (47, XXY).
- Cri du chat syndrome, or “cat cry” syndrome (46, XX or XY, 5p-).
Diseases can also occur due to chromosomal translocation in which portions of two chromosomes are exchanged.
This type of genetic disorder is caused by mutations in the non-nuclear DNA of the mitochondria.
Mitochondria are round or rod-like organelles involved in cellular respiration and are found in the cytoplasm of plant and animal cells.
Each mitochondrion can contain 5 to 10 circular pieces of DNA. Since ovules, but not sperm cells, maintain their mitochondria during fertilization, mitochondrial DNA is always inherited from the father.
Examples of the mitochondrial disease include:
- Eye disease is called the hereditary optic atrophy of Leber.
- A type of epilepsy called MERRF means myoclonic epilepsy with torn red fibers.
- A form of dementia called MELAS for mitochondrianemia, lactic acidosis, and episodes of apoplexy.
What is the human genome?
The human genome is the whole “treasure of human inheritance.” The human genome sequence obtained by the Human Genome Project, completed in April 2003, provides the first holistic view of our genetic heritage.
The 46 human chromosomes (22 pairs of autosomal chromosomes and two sex chromosomes) among them harbor nearly 3 billion base pairs of DNA containing approximately 20,500 genes encoding proteins.
The coding regions make up less than 5% of the genome (the function of all remaining DNA is not clear), and some chromosomes have a higher gene density than others.
Most genetic diseases are the direct result of a mutation in a gene.
However, one of the most challenging problems ahead is to elucidate further how genes contribute to diseases that have a complex pattern of inheritance, such as in cases of diabetes, asthma, cancer, and mental illness.
In all these cases, no gene has the power of yes/no to say whether a person will develop the disease or not.