A mutant recessive allele, such as the allele that causes sickle cell anemia (see Figure below and the link that follows), is not expressed in people who inherit just one copy of it. These people are called carriers . They do not have the disorder themselves, but they carry the mutant allele and can pass it to their offspring. Thus, the allele is likely to pass on to the next generation rather than die out. http://www.dnalc.org/resources/3d/17-sickle-cell.html
Mike was packing to go to the hospital again. It seemed like he was always in the hospital. Neither Mikes mom or dad had cystic fibrosis but Mike had inherited it from both of them.
Is the allele for cystic fibrosis likely to be passed on or die out?