What is genotype?

• A. A single gene's sequence
• B. The genetic makeup of an organism (TT, Tt, tt)
• C. The observable trait
• D. The set of alleles only on the X chromosome

Answer: (B)

Genotype is the genetic makeup of an individual for a particular gene or set of genes—the two alleles that an organism has at that locus. Since diploid organisms carry two copies of each gene, the genotype describes which two alleles are present, such as TT, Tt, or tt. This is different from what you can see (the trait) because the trait is the phenotype, while the genotype is the actual genetic composition that can determine that trait. The examples TT, Tt, and tt illustrate how the same gene can have different allele combinations, which, along with dominance/recessiveness, influence the expressed trait.