Final answer:
The characteristic 'Homozygous dominant for plant size' is a genotype, not a phenotype. Two individuals with the same phenotype might have different genotypes; for example one may be homozygous dominant (TT) and the other heterozygous (Tt) but both will be tall since 'T' is the dominant allele.
Step-by-step explanation:
Genotype vs Phenotype
The characteristic 'Homozygous dominant for plant size' is a genotype. This refers to the genetic makeup of an organism, specifically that it has two dominant alleles for a particular trait. Genotypes are the combination of alleles that an organism possesses, whereas phenotypes are the observable traits that result from the interaction of these alleles. For example, if a pea plant is homozygous dominant (TT) for plant size, its genotype is TT and its phenotype is tall.
An allele is a version of a gene that can vary between individuals. The type of allele that only affects the phenotype when in the homozygous condition is known as a recessive allele. It is only when two recessive alleles (tt) are present that the corresponding phenotype will be displayed. A tall, green plant that is homozygous for each trait, with 'T' being the tall allele and 'G' being the green allele, would have a genotype of TTGG and the phenotype of tall and green.
It is important to note that having a certain phenotype does not necessarily mean having the same genotype. Two individuals with the same phenotype may have different genotypes; one may be homozygous dominant (TT) while the other may be heterozygous (Tt) for a given trait. The phenotype will be the same (tall), even though the genotypes are different because the dominant allele 'T' masks the presence of the recessive allele 't' in the heterozygous individual.