recessiv

Recessive refers to a mode of genetic expression in which a trait or allele is phenotypically visible only when two copies are present. In most diploid organisms, genes occur in pairs, with one allele inherited from each parent. A dominant allele masks the effect of a recessive allele in a heterozygous individual, so the recessive trait is typically observed only in a homozygous recessive state (two recessive alleles). The term is used in several languages, where it is often written as recessiv or a close variant.

Recessive inheritance can involve autosomal or sex-linked genes. Autosomal recessive conditions require two copies of the recessive allele and often appear when both parents are carriers. Common examples include cystic fibrosis and phenylketonuria. X-linked recessive conditions, such as hemophilia and red-green color blindness, tend to affect males more frequently because males have only one X chromosome; females can be carriers.

Carriers carry one recessive allele and usually do not show the trait. Genetic tests and family history help determine carrier status. The chance of an affected child when both parents are carriers is 25% per pregnancy, 50% for a carrier child, and 25% for a non-carrier child. Penetrance and expressivity can influence whether the trait is observed even in homozygotes, or how clearly it is expressed.

Understanding recessive inheritance is important in medical genetics, breeding, and population genetics for assessing risk and guiding counseling. Notes: the term is used across languages, with closely related terms in German, Dutch, and Scandinavian languages.