The
Polyposis
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What is Familial Adenomatous Polyposis (FAP)?

FAP is an inherited condition which mainly affects the large intestine (also known as the large bowel or colon and rectum). A diagram of the intestine together with an explanation of how it functions can be found on page 6.

People with FAP develop many polyps (which are like small cherries on stalks) inside their large bowel. There are many different types of polyps but these particular polyps are called adenomas (the "adenomatous" in FAP). An adenoma can in time turn into a cancer which is why it is so important to make sure anyone at risk of inheriting FAP is examined. FAP is a serious condition unless detected early when it can be treated.

The faulty gene
that causes FAP is carried
on chromosome number 5.



What causes FAP?

FAP is a genetic condition, that is to say it is caused by a single faulty gene. The FAP gene is one of many thousands of genes carried on our chromosomes which determine our physical characteristics (our hair and eye colour, the shape of our nose, our blood type and the number of fingers and toes we have). Genes carry the information required to determine these characteristics as a chemical code in the form of DNA (deoxyribonucleic acid).

Sometimes mistakes occur in the DNA code so that genes carry wrong information. FAP occurs when the genetic recipe is wrong.

For example, consider a recipe. For a ham sandwich you need bread, butter and ham. If when writing the recipe the H in ham was changed to J the end result would be a jam sandwich. Just a minor change but the end result is very different.

When the FAP gene carries the correct information, the colon is protected from developing polyps. When the FAP gene is faulty (so that incorrect information is carried) this protection is lost and the colon develops many polyps.

What is the chance of inheriting FAP?

FAP is usually inherited from a parent who has the condition. Each child, boy or girl, born to a person with FAP has a 50:50 chance of inheriting the gene that causes it. A 50:50 chance is the same as the chance of getting a head or a tail when you toss a coin. This is known as an autosomal dominant mode of inheritance.

If a person has not inherited the gene that causes FAP then that person’s children will not be at any increased risk of getting polyposis.

Sometimes a person may be affected even though both parents have normal copies of chromosome 5. This is because mistakes can occasionally occur in the DNA as the chromosome is transmitted from parent to child.

In cases where this appears to have happened it should be confirmed. Parents, brothers and sisters should be screened. The children of a new person with FAP will have a 50:50 chance of inheriting the affected gene.

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