Final answer:
An organism's genotype is the combination of alleles for a gene that it inherits from its parents. This genotype, represented by letter combinations such as TT, Tt, or tt, can be homozygous or heterozygous, denoting its genetic make-up for a specific trait.
Step-by-step explanation:
An organism's genotype is an abbreviation for the genetic make-up for one gene, identifying both alleles. The genotype consists of two alleles, one inherited from each parent, and is represented by letter combinations like TT, Tt, and tt.
For a gene coded by a single locus, the alleles may be either homozygous (the same allele from both parents, either dominant or recessive, like TT or tt) or heterozygous (different alleles from each parent, like Tt). These letter combinations determine the organism's genetic make-up for a specific trait.
In genetic nomenclature, dominant alleles are denoted by uppercase letters, while recessive alleles are denoted by lowercase letters.
For instance, in pea plants, homozygous dominant for violet flowers is designated as VV, homozygous recessive for white flowers as vv, and heterozygous violet flowers as Vv. The phenotype is the observable traits that result from the interaction of these alleles.