Cystic fibrosis is a genetic autosomal recessive disorder that is inheritable and can cause damage to the lungs, digestive system and the reproductive system. It causes a mutation in the CFTR gene due to which a faulty protein is produced that cannot perform its function. The cystic fibrosis transmembrane conductance regulator (CFTR) protein is needed for controlling the movement of salts into and out of the cells. If it does not work properly, then the chloride ions cannot move out of the cells, due to which thick mucus develops as it is not hydrated by water. The mucus then hinders different functions of pancreas, lungs and other organs.
If the parents do not have cystic fibrosis but they still have a child with cystic fibrosis, this means that both parents are the carriers of the recessive allele that causes this disease. Carriers mean that they will have one dominant normal allele (A) and the other recessive defective allele (a); hence, being heterozygous (Aa). As it is an autosomal recessive disorder, it means to express its phenotype; it needs two recessive alleles in the genotype being homozygous recessive (aa). There is a 25% chance that a child will be born with the disease.
The monohybrid cross below explains this concept: