Which genotype represents a homozygous organism?

A homozygous organism has two identical alleles for a specific gene. This means that both inherited copies from the organism's parents are the same, whether they are dominant or recessive.

In the given choices, the genotype represented by two capital T's, TT, is a clear example of homozygosity as it consists of two identical dominant alleles. Therefore, an organism with the TT genotype expresses that particular trait consistently.

On the other hand, Tt represents a heterozygous genotype, where there is one dominant and one recessive allele, illustrating genetic variation rather than homogeneity. The combination TtTt seems to suggest a genetic combination, but it doesn’t illustrate a single genotype properly and rather indicates a mix. Lastly, the genotype tt represents a homozygous condition as well but is comprised of two identical recessive alleles. However, since the question asks for the representation of a homozygous organism with one clear answer, TT stands out as the representation of homozygosity due to its simplicity and clarity regarding dominant traits.

In summary, the genotype TT perfectly fits the definition of a homozygous organism since it consists solely of identical alleles.