Types of Genetic Disorders II: Definitions & Symptoms

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  • 0:01 Good Genes Gone Bad
  • 1:00 Hemophilia
  • 3:00 Muscular Dystrophy
  • 4:19 Phenylketonuria
  • 5:36 Lesson Summary
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Lesson Transcript
Instructor: Adrianne Baron

Adrianne has a master's degree in cancer biology and has taught high school and college biology.

We are going to discuss hemophilia, muscular dystrophy (MD), and phenylketonuria (PKU) in this lesson. We will look at the genes affected for each disorder. The symptoms that result and inheritance patterns will also be covered.

Good Genes Gone Bad

Our genetic code is the set of genes that determines almost everything about us. You can think about our genetic code as the instructions for creating us.

They are responsible for our hair color, height, eye color, etc. Most people are very content with everything that their genes have dealt them in life. This occurs when our genes are healthy and carry out everything that they were designed to do.

There are instances when genes don't work quite like they are supposed to. The most common reason for our genes not functioning properly is that there is a mutation. A mutation is an alteration in the structure of a gene. Mutations in our genes can cause a change in our appearance, the functioning of our bodies, or both. So, in other words, if the genes are mutated, then the instructions for creating us change, and the end result is different.

We are going to discuss a few of the diseases that result from mutations in our genes.


I'm sure you have scraped your knee or cut your finger before in life. This is a minor event for most people. The injury happens, you bleed a little bit, the bleeding stops, and then a scab forms.

No big deal, right? It is for people with hemophilia. Hemophilia is a disease that allows bleeding to occur for an extended time after an injury. This disease happens as a result of a mutation in the coagulation factor genes. Coagulation factors are proteins that are responsible for causing your blood to clot and form scabs after injury.

When mutations occur in the coagulation factor genes, then the instructions for making the coagulation factors are changed, and you end up not having functioning coagulation factors. Once this happens, then you aren't able to form clots like you should, and you continue to bleed after an injury.

There are two coagulation factors that may be mutated to cause this disease. A mutation in the gene for coagulation factor VIII causes hemophilia A while a mutation in the gene for coagulation factor IX causes hemophilia B.

You may be wondering: When and how does this mutation occur? This is usually a congenital disorder, which means that you are born with it. The genes for the coagulation factors are found on the X chromosome. The disease is seen almost exclusively in males since they only get one copy of the X chromosome. If the one X chromosome has the mutated copy, then males have the disease. Females rarely, if ever, get it because they have a second X chromosome that will likely have a normal set of genes for coagulation factors.

There are some instances when a person isn't born with the disorder and it develops later in life. This happens when the body thinks the coagulation factors are foreign, like bacteria, and mounts an immune response to attack them. We refer to this as acquired hemophilia.

Muscular Dystrophy

There are some activities that most of us don't give much thought to being able to do. Things like swimming, running, and exercising are rather easy to do since we decide to them. There is a subset of people that have a disease known as muscular dystrophy that really wish they were able to easily participate in these activities.

Muscular dystrophy, abbreviated as MD, is a genetic disorder that causes weakening of the skeletal muscles. These are the muscles that control movement. The disease may also affect the muscles of the heart and the ones that control breathing, in rare cases. When a person shows symptoms of muscle weakness, the muscles involved and how severe the weakness is depends on which type of disorder a person has.

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