What Is Life? 2nd Edition by Jay Phelan

Exercise 4

Two healthy parents have two children, both of whom have a degenerative muscular disorder called SMA (spinal muscular atrophy).
a) Explain how SMA must be inherited and what the genotypes of the parents must be.
b) The parents wish to have a third child and are contemplating adopting versus having a biological child. They seek the guidance of a genetic counselor and ask what the probability of having another child with SMA would be. What does the genetic counselor tell them? Explain.