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. |