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Hereditary diseases are genetic disorders produced by an alteration in the normal DNA sequence of a cell. You may remember from biology class that DNA is an acronym for deoxyribonucleic acid. The primary constituent of genes and chromosomes, DNA carries the genetic information or instructions, if you will, that human beings need to develop, survive, and reproduce. DNA instructions come in a code that cells can read. This code tells cells how to combine amino acids, of which there are twenty, to produce proteins. Human beings need proteins to manage the functions of their bodies and to provide support for and structure to their cells.

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With the unlocking of the human genome comes the revelation that nearly all diseases have a genetic component of either genetic mutations or chromosomal abnormalities. Mutation can occur in a single gene or multiple genes. In most cases, mutated genes are not inherited. They are acquired randomly at some time in a person’s life when sparked by an environmental factor/s, and the mutations are present in only specific cells of the body, not in every cell. Environmental factors such as ultraviolet radiation or exposure to carcinogens can cause a gene to mutate.​


There are three categories of genetic disorders:

1. Monogenetic disorders occur when a mutation in a single gene is passed from parent to child and is present at birth. Additionally, each successive generation of children can inherit the mutated gene either through an autosomal dominant pattern of inheritance or an autosomal recessive pattern.

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An autosomal recessive disorder occurs when each parent contributes one mutated gene to the same individual. It is rare for a carrier to have his/her own health affected even though he/she has one recessive, mutated gene because the carrier also has one normal gene, and the normal gene is dominant.

An autosomal dominant disorder requires the contribution of only one copy of a mutated gene from either parent (not both) to the same individual. One of the parents may also have the disease.  

Scientists currently estimate that over 10,000  human diseases are known to be monogenic. Some of the more common ones include:

Sickle cell anemia

Marfan syndrome

Huntington's disease


Tay-Sachs disease

Hemophilia A & B

Fragile X syndrome

Duchenne and Becker muscular dystrophy

Thalassemia (Alpha & Beta)

Amyotrophic lateral sclerosis (ALS, Lou Gehrig’s disease)


Cystic fibrosis

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2. Multifactorial Inheritance Disorders occur when a combination of inherited mutations in multiple genes act together with environmental factors. Alcoholism, heart disease, obesity, mental illness, and Alzheimer's disease are examples. Experts believe that Parkinson’s disease, one of the most common neurodegenerative disorders in human beings, is the result of environmental factors acting on genetically predisposed individuals as they age.


3. Chromosomal Disorders are caused by an excess or deficiency of certain genes located on specific chromosomes or by structural changes within the chromosomes themselves. For example in Down syndrome, there is an extra copy of chromosome 21, and in Prader-Willi syndrome, there is an absence or non-expression of a group of genes on chromosome 15. 


Thus, an altered gene/s, a chromosomal excess or deficiency, or a combination of genes that predispose an individual to illness when coupled with exposure to particular environmental factors are the three ways in which parents pass on hereditary diseases to their children.

Articles on this site are written after careful research by the author of Twisting Legacy. Even so, the author is not an expert on any of the subjects. Please do your own research if you have questions about the authenticity of the information in the articles on this website.  

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