The correct definition of an organism's genotype is:
The set of alleles an organism has for a trait.
The other statements refer to concepts related to genetics but do not define genotype directly. Here's a brief clarification:
- An allele that is expressed whenever it is present refers to a dominant allele.
- A trait that is expressed in a homozygous individual usually describes a recessive trait, which is only expressed when both alleles are the same.
- An observable trait is referred to as a phenotype.
So, in summary, genotype specifically pertains to the genetic makeup, while the other terms relate to alleles and traits.