The gene for cystic fibrosis is a recessive trait. this disorder causes the body cells to secrste large amounts of mucus that can damage the lungs, liver, and pancreas. if one out of 20 people is a carrier of this disorder, why is only one out of 1,600 babies born with cystic fibrosis?

The reason this is so different is because those that are carriers (1 in 20) are heterozygous and to have a child with CF, they have to have a child with another carrier and then have the unlucky occurrence that their child will end up homozygous recessive.

So, 2 parents are Cc
C= normal CFTR (the gene at question here - Cystic Fibrosis Transmembrane Receptor)
c = mutated CFTR

Cross: Cc x Cc
You get: CC (1); Cc (2) and cc (1)there is only a 0.25 chance that each time you have a child that it will be diseased (cc). Highest chance that the child is a carrier (Cc - 0.50) and similar low chance to no longer carry the recessive damaged allele (CC - 0.25)