carrier
A carrier is someone who quietly holds a recessive disease allele without showing the disease. They have one copy of the recessive allele and one ordinary working copy; the working copy is enough to keep them healthy, but they can still pass the recessive allele on to a child.
Carriers are the hidden conduit of recessive inheritance. A recessive condition usually appears only when a child inherits the recessive allele from both parents — and the most common way that happens is when both parents are unaffected carriers. Carrier status is invisible without testing, which is why recessive conditions can surface in families with no apparent history.
The term is sometimes stretched to female carriers of an X-linked recessive allele, who are usually unaffected because their second X compensates. More broadly, 'carrier' can even mean anyone harboring an allele not yet expressed — but its core, classic meaning is the healthy heterozygote for a recessive trait.
Carrier screening before or during pregnancy can reveal that two healthy partners each carry a cystic fibrosis allele, giving each child a one-in-four risk.
Two healthy carriers, a hidden recessive allele each — and a 25% risk in every pregnancy.
Being a carrier is normal and common — every person carries several recessive disease alleles. It matters mainly when two carriers of the same condition have children together.