How Can A Child Inherit A Disease From Two Normal Parents?
A child inherits two copies of each gene: one from the mother and one from the father. In most cases, genetic diseases are said to be “recessive,” which means that if just one of the two copies is defective, the other copy will keep the child healthy.
A person who inherits one defective copy and one normal copy of a gene is a “carrier” of the mutation. Even though they are unlikely to get sick, they have a fifty-fifty chance of passing the mutation along to each of their children. This is one reason why some diseases seem to disappear in one generation, only to reappear in later generations.
Just as a children may inherit their hair or eye color from their parents, they can also inherit genetic diseases. If a child inherits just one mutated copy of the gene he or she is a "carrier" of the genetic disease. If there are two defective copies the child may be affected by the disease.