Recessive

In genetics, "recessive" describes an allele or trait that is only expressed when an individual has two identical copies of that allele (no dominant allele is present). If a dominant allele is present, it will mask the recessive allele's effect, so the recessive trait does not show.

• Someone with one recessive allele and one dominant allele for a trait is a carrier of the recessive trait - they usually do not show the trait but can pass the recessive allele to children.
• Recessive traits can appear to skip generations: two unaffected carrier parents can have a child who shows a recessive trait (each child of carrier parents has a 25% chance of being affected).
• Recessive does not mean rare. For example, having five fingers is actually due to recessive alleles and is very common, whereas the trait for six fingers (polydactyly) is caused by a dominant allele but is rare.

• Pedigree clues for a recessive trait: the trait may pop up in a child even if neither parent shows it (unaffected parents having an affected child suggests recessive inheritance). Often, these parents are carriers of a recessive allele.
• In Punnett square problems, crossing two heterozygotes (carriers) typically yields a 3:1 ratio of phenotype (75% exhibit the dominant trait, 25% the recessive trait). That 1 in 4 showing the recessive trait is a hallmark of recessive inheritance.
• Be ready to recognize autosomal recessive patterns (e.g., a disorder appearing in siblings born to unaffected parents, with equal probability in sons and daughters). Classic examples of recessive conditions include cystic fibrosis and sickle-cell anemia, which occur only when both alleles are the recessive variant.