What is a Cell?
Cells are the building blocks of all living things. Our bodies are full of trillions of cells that do many things - they provide the body's structure, use nutrients from food, convert those nutrients into energy as well as perform specialized functions.
Cells also contain our body's hereditary material and can make copies of themselves.
What is DNA?
DNA (also known as deoxyribosenucleic acid) is the hereditary material found in almost every cell of our body. Most DNA is found in the nucleus of the cell but a small amount can be found in the mitochondria. If the DNA is in the mitochondria, it is called "mitochondrial DNA."
DNA is composed of four chemical bases:
These DNA bases pair up with each other A-T and C-G to form "base pairs" which are also attached to a molecule of sugar and phosphate. This unit is called a nucleotide and are arranged in a double helix, a spiral, and determines our characteristics.
DNA can make copies of itself using the pattern of the double helix as a guideline for replication. This is critical for cell division.
What's A Gene?
A gene is the physical and functional unit of hereditary, made up of DNA, which acts as blueprints to make protein molecules. Our genes vary from a few hundred DNA bases to over 2 million bases. It's estimated that we humans have somewhere between 20,000 and 25,000 genes.
We each have two copies of a gene - one from each parents. Most genes are the same in all people - although a small number of genes are slightly different among people, giving us each distinguishing physical characteristics.
What's a Gene Mutation?
A gene mutation is a permanent alteration in the DNA sequence of a gene. Gene mutations may range in size from a single DNA strand to a large part of a chromosome.
How Do Gene Mutations Occur?
Genes can mutate in various ways.
- Inherited Genetic Mutations (also called hereditary mutations or germline mutations) are passed from parent to child and is present through a person's life in almost every cell of the body.
- De Novo Mutations - Mutations that occur only in an egg or sperm cell, or those that occur immediately following fertilization.
- Acquired Mutations (also called Somatic Mutations) occur in the DNA of certain cells at some point during a person's life. These acquired changes can be caused by environmental factors or if a mistake occurs during cell division. Acquired mutations in cells other than sperm or egg cells (somatic cells) cannot be passed on to offspring.
- Mosaicism is another type of genetic mutation that happens when mutations occur in a single cell of a developing embryo. As the cells grow and divide, the developing fetus will have some cells with genetic mutations and some cells without the mutation.
How Do Genetic Mutations Affect Us?
To properly function, each cell requires thousands of proteins to do their jobs in the right places at the right times. Sometimes, a genetic mutation can prevent a protein from properly functioning. A mutation can cause a protein to malfunction or to be missing.
When a genetic mutation affects a protein that is vital to the body, it can cause what is known as a genetic disorder.
What Are Genetic Disorders?
A genetic disorder is a disorder caused by an abnormality in a person's DNA. These abnormalities can range from a tiny mutation in a single gene to the addition (or deletion) of a whole chromosome - or set of chromosomes.
Some genetic disorders, including certain cancers, are caused by a mutation in a gene - or group of genes - in an individual's cells. These mutations can occur randomly or due to environmental exposure to toxic substances.
Some genetic disorders are inherited, or passed down through a family. The mutated gene is passed down so that each generation of child may inherit the gene that causes a disease.
However, most genetic disorders are known as Multifactoral Inheritance Disorders, which mean that these conditions are caused by a combination of inheritance of gene mutations that work with environmental factors to cause the disease.
Geneticists group genetic disorders into three categories described in further detail below.
These categories include: Monogenic Disorders, Multifactoral Inheritance Disorders and Chromosomal Disorders.
What Are Single Gene (Monogenetic) Disorders?
Single Gene Disorders (alternately known as Monogenetic Disorders) are the result of a mutation that causes a protein product of a single gene to be altered or deleted. The mutated gene may be present on one or both chromosomes (we receive a single chromosome from each parent, forming a pair of chromosomes).
These monogenic disorders are fairly rare in comparison to multifactoral inheritance disorders (listed below). It's estimated that there are over 4,000 diseases caused by single gene defects.
There is further breakdown of Monogenetic Disorders into "dominant" and "recessive" diseases.
Autosomal Dominant Diseases - these single gene disorders are caused by presence of one mutated copy of the gene. A person with an autosomal dominant disease typically has one affected parent. The chance for each child to inherit an autosomal dominant disease is fifty percent.
Examples of Dominant Diseases include Huntington's Disease and Marfan Syndrome.
Autosomal Recessive Diseases - two copies of the mutated gene must be present for a person to be affected by an autosomal recessive disease. An affected person generally has two unaffected parents (also known as carriers of a disease) who each carry a copy of the mutated genes.
Two unaffected people who each have one copy of the mutated gene have a 25% chance per pregnancy to have a child affected with the disorder.
Examples of Recessive Diseases include cystic fibrosis and Tay Sachs.
Read more about Cystic Fibrosis.
Read more about Tay-Sachs.
X-Linked Dominant Disorders - these are disorders caused by mutations in the genes on the X chromosomes. Only a few diseases have this inheritance pattern. While men and women can both have X-Linked Dominant Disorders, more males are severely affected.
Examples of X-Linked Dominant Disorders include Rett Syndrome, and Klinefelter Syndrome.
X-Linked Recessive Disorders - these are disorders that are also caused by mutations in genes on the X chromosome.
Men are more readily affected than women, the chance of passing on the disorder to offspring differs between men and women. The sons of a father with an X-Linked Recessive Disorder won't be affected, and his daughters will carry a single copy of the mutated disease.
A woman who is a carrier for X-Linked Recessive Disorders has a 50% chance of having a son who is affected and a 50% chance that her daughter will become a carrier (carry a copy of the mutated gene).
Read more about Hemophilia.
Read more about Muscular Dystrophy.
Y-Linked Disorders - these disorders are caused by mutations on the Y chromosome. As all males inherit a Y chromosome from their fathers, each son of an affected father will have the disorders. As females inherit an X chromosome from their father, they will never be affected.
Mitochondrial Disease (also known as maternal inheritance) - as the cells from a woman's ova are what contribute mitochondrial DNA to the developing embryo, only mothers can pass Mitochondrial Disease onto their children.
Examples of Mitochondrial Disease include Leber's hereditary optic neuropathy.
What Are Chromosomal Disorders?
Chromosomal Abnormalities are disorders that involve entire chromosomes - or large segments of them - that are missing, duplicated or altered in some way.
Down Syndrome, a Trisomy, for example, is caused by an extra copy of chromosome 21.
Read more about Down Syndrome.
What Are Multifactorial Inheritance Disorders?
Multifactoral disorders can be complex and polygenic disorders, which means that these diseases are likely the effects of multiple small genetic variations in combination with lifestyle and environmental factors. Multifactoral disorders often cluster in families but there's no clear-cut pattern of inheritance which means that determining the risk for developing these disorders is diffucult.
Examples of Multifactorial Inheritance Disorders include heart disease and diabetes.
Read more about heart disease.
Read more about diabetes.
Additional Resources for Genetic Disorders:
Genetics Home Reference - US government site that provides consumer-friendly information about the effects of genetic variations on human health.
Genetic Disorders Library - a science and health education program located in the midst of the bioscience research being carried out at the University of Utah. Our mission is making science easy for everyone to understand.
National Human Genome Research Institute - began as the National Center for Human Genome Research (NCHGR), which was established in 1989 to carry out the role of the National Institutes of Health (NIH) in the International Human Genome Project (HGP). The HGP was developed in collaboration with the United States Department of Energy and begun in 1990 to map the human genome. In 1993, NCHGR expanded its role on the NIH campus by establishing the Division of Intramural Research to apply genome technologies to the study of specific diseases.