Recessivity

Recessivity is a term used in genetics to describe how certain alleles influence an organism’s phenotype. A recessive allele’s trait is typically masked by a dominant allele when an individual has one copy of each (a heterozygous genotype). The recessive phenotype usually appears only if the organism has two copies of the recessive allele (a homozygous recessive genotype). By contrast, a dominant allele determines the phenotype in heterozygotes.

In classical Mendelian inheritance, recessive traits follow predictable patterns. When two carriers (heterozygotes) mate, there is a 25% chance of producing an offspring with two recessive alleles and the recessive phenotype, a 50% chance of a carrier child, and a 25% chance of a child with two dominant alleles. Examples of recessive human conditions include cystic fibrosis, phenylketonuria, and albinism. Many recessive traits arise from loss-of-function mutations, where a single functional copy of the gene is insufficient to produce the normal trait.

Not all alleles act strictly recessively or dominantly. Some mutations show partial effects, or require interactions with other genes (epistasis), and a recessive allele may have effects under certain conditions or in specific genetic backgrounds. Dom­inance itself is not an intrinsic property of a gene but a description of how alleles interact in the phenotype.

In population genetics and medical genetics, recessivity affects carrier screening, inheritance risk assessment, and counseling, especially where consanguinity or population structure increases the likelihood of homozygosity for recessive alleles.