While natural selection often favors traits that enhance survival and reproduction, some deleterious traits—those that negatively affect an organism’s health or fitness—persist in the gene pool. This can happen for several reasons, including the way genes are inherited and expressed.
A harmful mutation does not always disappear immediately from a population. Many genetic diseases are recessive, meaning they only manifest in homozygous (homo = same or identical) individuals who inherit two copies of the mutated allele, one from each parent. Heterozygous (hetero = different) carriers, who have one normal and one mutated allele, do not exhibit the disease but can pass the mutated allele on to their offspring. This allows harmful recessive alleles for genetic diseases like cystic fibrosis to persist in the population at low frequencies because carriers do not face a selective disadvantage, even though those same alleles are selected against in individuals with two copies.
Click on each of the figures below to learn more.
A carrier has only one recessive allele. They will not exhibit the disease.
A child that inherits one recessive allele from each parent will exhibit the disease.
Children who inherit one unaffected and one affected allele will not exhibit the disease, but will be a carrier for future generations.
A child who inherits two unaffected alleles will neither exhibit the disease nor be a carrier.