autosomal recessive
Think of a door that needs two keys turned at once to open. In autosomal recessive inheritance, the trait appears only when both copies of an allele on a numbered chromosome are the altered version; a single working copy is enough to keep things normal.
Most affected people inherit one recessive allele from each parent. The parents themselves usually carry one recessive and one ordinary allele, so they show no trait but are carriers. When two carriers have children, each child has a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of inheriting neither recessive allele.
Because the trait skips silently through carriers, it can appear suddenly in a child whose parents and grandparents seem unaffected, often as a horizontal cluster among siblings rather than down the generations. The chance of two carriers meeting rises when parents are related (consanguinity) or come from a small population where a recessive allele is common.
Cystic fibrosis is autosomal recessive: a child must inherit a faulty CFTR allele from each parent, and the two carrier parents typically have no symptoms.
Two recessive alleles needed; carrier parents are silent, and the 25% risk repeats each pregnancy.
Two unaffected carrier parents have a 25% risk in each pregnancy. Because carriers feel no effects, recessive traits often surprise families with no prior history.