Jeff's genome contains a recessive gene for cystic fibrosis. jeff is said to be a of the gene for cystic fibrosis.

Jeff is said to be a carrier of the gene.
Recessive genes are those that appear in an organism's phenotype when there are two copies of the allele present. Some organisms have one copy of the dominant allele and one copy of the recessive allele, as is the case with Jeff. These organisms carry the allele and may pass it down to their offspring.

From a high concentration to a low concentration.