When allele combinations are expressed and you can see the trait what are they called ?

Air pollution is only caused by human activity, select the best answer from the choices provided ot of true or false

Most of the fossils found in virginia are in the coastal plain, valley, ridge, and appalachian plateau regions, best suggesting that these geographic provinces a) were once covered by seawater. b) are the oldest parts of virginia. c) have a great deal of karst topography. d) have large amounts of sedimentary rock.

Why has two different alleles for the same trait?