HOW IS THIS DISEASE PASSED ON?
The metabolic disease phenylketonuria (PKU) is transferred to people through the genes they inherit from their parents. There have been cases of metabolic diseases being caused from the over consumption of a certain food and also via exposure to drugs that have caused a dormant gene to become active. This is exceptionally rare though and diseases such as PKU are detected at birth.
The way the disease is passed on is determined by where the faulty gene is located within the chromosomes of the ‘carriers’. The two main hereditary paths (you may remember from school) are through dominant and recessive genes.
PKU is an example of an autosomal recessive disorder, in which PKU is only inherited if both the mother and father are carriers of the recessive gene. Below are two diagrams that illustrate how the faulty gene can be passed from one generation to the next:
Diagram one (unaffected carrier father and mother)
Where the faulty gene is present in both parents: This means that there will be a 25% chance of a child being affected, a 50% chance of a child being an unaffected carrier and 25% chance the child will be neither affected or a carrier.
Diagram two (unaffected carrier father and non-carrier mother):
When only one parent carries the recessive faulty gene, there is a 50% chance of a child being an unaffected carrier and a 50% chance that the child is neither affected or a carrier. PKU can not be passed on this way though but other metabolic diseases such as Achrondoplasia can.
The only way to discover if you do carry the defective genes is to see a genetic counsellor. Trying to ‘work out’ who may be a carrier through a family tree will not give an accurate answer.