Recessive

Recessive refers to a mode of genetic inheritance in which a trait is expressed only when two copies of a recessive allele are present. In diploid organisms, each gene has two alleles; a recessive allele typically produces its phenotype only when paired with another recessive allele. A dominant allele, in contrast, can express its trait with just one copy and can mask the effect of a recessive allele in a heterozygote.

In autosomal recessive inheritance, individuals with two recessive alleles (homozygous recessive) express the trait, while heterozygous individuals carry one recessive and one dominant allele and usually show the dominant phenotype. These carriers typically do not have the disease but can pass the recessive allele to offspring. Per pregnancy, two carriers have a 25% chance of an affected child, a 50% chance of a carrier child, and a 25% chance of a child who is unaffected and not a carrier.

Many human recessive disorders are autosomal, including cystic fibrosis, phenylketonuria, sickle cell disease, and Tay-Sachs disease. Recessive conditions can also be X-linked, where they are more likely to affect males; examples include hemophilia A and Duchenne muscular dystrophy. Female carriers of X-linked recessive mutations may transmit the allele to their sons or daughters.

Recessive traits can persist in populations because carriers do not always show disease symptoms, and allele frequencies are influenced by population genetics, including mutation, selection, and genetic drift.