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The Story of any Recessive Disease
The gene associated with PXE is called ABCC6.  Every living person has the gene called ABCC6.  It is not true that people affected by PXE have the “PXE gene”; instead they have a change (mutation) in the gene associated with PXE, the ABCC6 gene.

All of our genes come in pairs, so we all have two copies of every gene.  Some genes are dominant, meaning that a change (mutation) in just one copy of the gene will cause a sign or a symptom.  Some genes are recessive, meaning that both genes must have a change (mutation) in order to cause a sign or symptom. PXE is a recessive trait, so an individual will only have PXE if they have changes (mutations) in both copies of ABCC6.  A person is a carrier of PXE if they have a change (mutation) in only one copy of ABCC6.

Examples
In the examples below, 'a' means there is a change in ABCC6, while 'A' means there is no change or mutation.  By drawing a small chart, called a Punnett square, we can see all the possible combinations of gene pairs.

Symbols used:
A = ABCC6 gene with no change or mutation
a = ABCC6 gene with a change or mutation

Possible combinations:
AA = not a carrier, doesn´t have PXE
Aa = carrier
aa = has PXE

	A	a
A	AA	Aa
A	AA	Aa

Result 1:
50% chance that a child will not have PXE or be a carrier (AA)
50% chance that a child will be a carrier (Aa)

	A	a
A	AA	Aa
a	Aa	aa

Result 2:
25% chance that a child will not have PXE or be a carrier (AA)
50% chance that a child will be a carrier (Aa)
25% chance that a child will have PXE (aa)