RECESSIVE

A recessive gene expresses itself only when there is no other type of gene present at that locus (spot on the genetic code or chromosome). For example, cystic fibrosis (CF) and Duchenne muscular dystrophy (DMD) are both recessive disorders. A CF child has the CF gene on both chromosome 7’s (and so is said to be homozygous for CF). The DMD boy has the DMD gene on his sole X chromosome (and so is said to be hemizgous for DMD).