When both parents are carriers, the chance is one in four that a child will inherit two of the recessive alleles, one from each parent, and develop disease. Although most recessive mutations are rare, a few, including those for cystic fibrosis and sickle-cell anemia, are fairly common in specific ethnic groups. To begin with, not all mutated alleles invariably lead to disease. Even with a dominant allele such as the BRCA1 breast cancer susceptibility gene , for instance, the risk of disease by age 65 is 80 percent, not 100 percent. Not only can different mutations in the same gene produce a wide range of effects in different individuals, as is the case with cystic fibrosis, but also mutations in several different genes can lead to the identical outcome, as is the case with some forms of Alzheimer's disease .
Some traits require simultaneous mutations in two or more genes.
And a phenomenon known as imprinting can determine which of a pair of genes, the mother's allele or the father's, will be active or silenced. Different mutations in the same gene can produce a wide range of effects.