Answer with Explanation:
This kind of "muscular dystrophy" is brought about by "spontaneous mutation." This means that the genes of Tracie's first born child mutated for no apparent reason. The mutation affected the function of the cells, thereby causing the disease.
It could also be that both Tracie and her husband (or probably one of them) has such a recessive condition. This means that the gene mutation wasn't apparent for them or for many generations, but it became apparent in their child when the child inherited the mutated gene.
Most of the time, muscular dystrophy is passed from generation to generation or is heritable. The case of Tracie's child is just one of the rare cases of "spontaneous mutation."
Thus, this explains the answer.
