Appendix 1
The following diagrams explain the chances, for each pregnancy, of a child inheriting SMA in different families. Please note the chances of inheriting SMA are the same for males and females; the sex shown for children and parents in the diagrams is purely illustrative.
Family 1
Both parents are carriers.
For each pregnancy, the chances are:
25% (1 in 4) child has both faulty copies of the SMN1 gene and will have SMA
50% (1 in 2) child has one faulty copy and one healthy copy of the SMN1 gene and will be a carrier
25% (1 in 4) child has two healthy copies of the SMN1 gene and will not be a carrier or have SMA.
Family 2
One parent is a carrier, the other parent does not have SMA and is not a carrier.
For each pregnancy, the chances are:
50% (1 in 2) child has two healthy copies of the SMN1 gene and will not have SMA and will not be a carrier
50% (1 in 2) child has one faulty copy and one healthy copy of the SMN1 gene and will be a carrier
Family 3
One parent has SMA; the other does not have SMA and is not a carrier.
For each pregnancy, the chances are:
100% (4 in 4) child has one faulty copy and one healthy copy of the SMN1 gene and will be a carrier
Family 4
One parent has SMA; the other is a carrier.
For each pregnancy, the chances are:
50% (1 in 2) child has two faulty copies of the SMN1 gene and will have SMA
50% (1 in 2) child has one faulty copy and one healthy copy of the SMN1 gene and will be a carrier
Family 5
Both parents have SMA.
For each pregnancy, the chances are:
100% (4 in 4) child has two faulty copies of the SMN1 gene and will have SMA
