~98% of parents of an affected child are carriers. Whereas, the rest of 2% that are not carriers have denovo mutation.
The pattern of inheritance is autosomal recessive in SMA. The risk of occurrence/recurrence if both the parents are carriers is 25%, while the risk of having an unaffected carrier child is 50%.
This means that in every pregnancy there is
1 chance in 4 (25% chance) that they will have a child who inherits both copies of the mutated gene. So, there will be no working gene product in the child and he/she will be affected by the condition.
1 chance in 4 (25% chance) that their child will inherit both copies of the working gene and will be unaffected by the condition
1 chance in 2 (i.e. 2 chances in 4; 50% chance) that their child will inherit the mutated gene and the working copy of the gene from each parent and he/she will be an unaffected carrier of the faulty gene, just like the parents i.e. carrier for the condition
Spinal muscular atrophy risk factor
Amyotrophic lateral sclerosis risk factors
Lesch-Nyhan syndrome pathophysiology
Ataxia risk factors
Fragile X syndrome risk factors
Tuberous sclerosis pathophysiology
Cerebral palsy pathophysiology
Bannayan-Riley-Ruvalcaba syndrome pathophysiology