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Genetics 101
  Inheritance Patterns

By Amanda Ewart Toland, PhD

Reviewed by Karen Hales, PhD
Last updated June 23, 2011

Many diseases occur as a result of mutations in certain genes. However, inheriting a gene with a mutation from only one parent doesn't mean that you are at risk for the disease. Everyone has two copies of most genes — one copy from their mother and one from their father. Sometimes it only takes one damaged copy of a gene to cause disease while other times it takes two. In fact, there are many different ways to inherit diseases and other traits.


Autosomal Recessive

In some cases, it takes two copies of a mutated version of a gene in order for to develop that trait. One example of this is hair color. If a person has one version of a hair gene that results in blond hair and one that results in brunette hair, the person will have dark hair. It takes two copies of the blond hair version of the gene in order for the person to have blond hair. In this pattern of inheritance, called autosomal recessive, both parents must pass on the particular gene trait (blond hair) in order for their child to have the trait.

People who only carry one copy of a recessive mutation are called carriers for that trait.
Hemochromatosis (an iron storage disorder) is inherited in an autosomal recessive fashion, requiring two mutated copies of the hemochromatosis gene in order for the person to have the disease. People who only carry one copy of a recessive mutation, such as the one that causes hemochromatosis, are called carriers for that trait. Although carriers do not develop the disease, they can pass it on to their children.



Autosomal Dominant

Sometimes, only one parent has to pass on the altered gene in order for that trait to be expressed. This type of inheritance is called autosomal dominant. Familial adenomatous polyposis (FAP), a disorder characterized by thousands of polyps and a predisposition to colon cancer, is inherited in an autosomal dominant fashion. Only one parent has to pass on the mutated gene in order for the child to inherit a risk for FAP.




Women have two X chromosomes, while men have one X chromosome and one Y chromosome.
The exception to the "everyone-has-two-copies-of-each-gene" rule are male sex chromosomes. Although females inherit two X chromosomes (one from each of their parents), males inherit one Y chromosome (from their father) and one X chromosome (from their mother). The X and Y chromosomes both contain many genes that the other chromosome does not. Because males have only one X chromosome, they only have one set of the genes that reside there.

This imbalance means that men and women differ in how they inherit certain diseases. For example, if a women inherits one copy of a recessive gene on an X chromosome, she probably won't develop that trait because she has a normal copy of the gene on her other X chromosome. She would have to inherit the recessive gene from both parents in order to develop that trait. However, if a man inherits one recessive copy of a gene on his X chromosome, he develops that trait because he has no additional copies of that gene. Thus, men are more likely than women to get many disorders associated with the X chromosome. Diseases that are caused by mutations on the X chromosome are called X-linked diseases. Hemophilia, a blot clotting disorder, and color blindness are X-linked disorders that are far more common in men than in women.



Mitochondrial Inheritance

Each cell has many mitochondria, whose job is to produce energy for the cell. Mitochondria are almost like cells within the cell because they have their own DNA, which is different from the rest of the cell's DNA. Because each cell has many mitochondria, it has many copies of mitochondrial DNA. Diseases can occur if numerous copies of mitochondrial DNA within a cell have been altered.

We inherit all of our mitochondria from our mother's egg; male sperm contain no mitochondria. Thus, disease genes carried on mitochondrial DNA can only be passed from mother to child. This rare pattern of inheritance is called mitochondrial DNA inheritance. Maternally Inherited Diabetes and Deafness (MIDD) is a disorder with this type of inheritance pattern.

Why do Mitochondria have their own DNA?

Scientists believe that long ago mitochondria were actually independent bacteria. These bacteria formed a close relationship with larger cells where each party provided something beneficial to the other (a relationship called symbiosis.) Over time, the bacteria transferred all but a small portion of their DNA over to the larger cells and were no longer able to live independently. It is thought that our mitochondria are the descendants of these bacteria and that our mitochondrial DNA is that last bit of DNA that the bacteria kept for themselves.




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