What is a carrier in?

Answer 1

Genetically, a carrier is someone who is carrying a defective allele but doesn't display any of it's characteristics.

For example:

If a male (with chromosomes X and Y) has a dud X chromosome, he will undoubtedly show the characteristic that come with it.

However, if a female (with chromosomes X and X) has the defective X chromosome, the symptoms will not be displayed due to the other, intact X chromosome. (assuming that the characteristic is recessive and not dominant)

The female is a Carrier

A carrier refers to someone who has a recessive gene that is not expressed (heterozygous). Such a trait is only expressed when an individual has two recessive genes (homozygous recessive). In humans, the gene for albinism is recessive. Any of us with normal pigmentation may be carriers for this trait. Parents who are both carriers will have a 1 in 4 chance of havibg an albino child. Genes (alleles) do not have to be "defective " to illustrate carriers. People with hitch-hiker's thumb are homozygous recessive. People with straight thumbs may be carriers (heterozygous) or have two dominant genes.

"Carrier" can also refer to a molecule which transfers an ion or functional group to another molecule, e.g. NADP is a hydrogen ion carrier, Coenzyme A is an acetate carrier.