Recessive allele

A recessive allele is an allele whose effect on the phenotype is hidden when it is paired with a dominant allele. Its trait shows only in individuals that are homozygous recessive, carrying two copies, as in the genotype aa. By convention it is written with a lowercase letter. Mendel noticed that recessive traits, such as green pea seeds and white flowers, vanished in the first generation of a cross and then reappeared in about a quarter of the next.

Many recessive alleles are versions of a gene that make little or no working protein. A single normal copy usually makes enough for the cell, so heterozygotes are healthy carriers who can still pass the allele on. When two carriers have children, each child has a one-in-four chance of inheriting two copies. Because carriers show no sign of the allele, recessive traits often seem to skip generations in a family.

Cystic fibrosis and sickle cell disease are autosomal recessive conditions. For genes on the X chromosome, males have only one copy, so recessive conditions such as hemophilia and red-green color blindness are far more common in males.