Recessiva

Recessiva refers to the concept of a recessive genetic trait or allele in genetics. In Romance languages, the word is the feminine form of the adjective meaning recessive, and in English-language genetics it is used to describe traits that require two copies of a particular allele to be expressed.

A recessive allele is usually masked by a dominant allele in heterozygous individuals. Expression of the associated trait occurs when an organism inherits two copies of the recessive allele (homozygous recessive). In X-linked recessive inheritance, males are more frequently affected because they have only one X chromosome, while females typically become carriers unless they inherit two affected copies.

Common human examples of recessive traits include cystic fibrosis (mutations in CFTR), phenylketonuria, and oculocutaneous albinism. In animal and plant genetics, recessive alleles can determine coat color, flower color, or other phenotypes, often revealed only when two recessive variants combine.

In population genetics, the frequency of recessive alleles influences the proportion of affected individuals according to Mendelian expectations, though real-world patterns can deviate due to factors such as nonrandom mating, selection, and genetic drift. Carriers play a central role in genetic counseling and screening, helping assess the risk of passing recessive conditions to offspring.

Limitations and complexities include incomplete penetrance, variable expressivity, and cases where multiple genes contribute to a trait, or where a recessive phenotype depends on regulatory elements or gene interactions beyond a single allele.