rezessive

Recessive, or rezessiv in some languages, describes a mode of genetic expression in which a trait or phenotype is observed only when two copies of the corresponding allele are present. In diploid organisms, an individual carries two alleles for each gene. A recessive allele is usually masked by a dominant allele and does not determine the phenotype unless the organism has two copies (homozygous recessive). Heterozygous individuals (one recessive and one dominant allele) typically show the dominant phenotype but can pass the recessive allele to offspring.

In autosomal recessive inheritance, two recessive alleles, one from each parent, are required for the trait to appear. A common example is cystic fibrosis caused by mutations in the CFTR gene. Carriers with one defective allele usually appear normal but can transmit the allele. When two carriers mate, there is a 25% chance of an affected child, a 50% chance of a carrier child, and a 25% chance of a child with two normal alleles (assuming full penetrance and no de novo mutations). Punnett squares are commonly used to illustrate this.

X-linked recessive traits are due to recessive alleles on the X chromosome. Because males have only one X chromosome, a single recessive allele can produce the phenotype in males, while females typically require two copies. Disorders such as red-green color vision deficiency and hemophilia are classic examples.

In population genetics, the recessive allele frequency is denoted q, with p + q = 1 for a given gene. Under Hardy-Weinberg equilibrium, phenotypic frequencies follow q^2 for recessive traits and p^2 + 2pq for dominant and carrier states. Real-world factors such as penetrance, expressivity, and selection can modify these patterns.