What is the genotype of a red-flowering plant in a species where red flower color is dominant?

In a species where red flower color is dominant, the presence of a red allele signifies that the plant will exhibit red flowers, regardless of the second allele's identity. This is due to the nature of dominance in genetics, where one allele can mask the effect of another.

When we consider the genotype of a red-flowering plant, the key aspect is that it has at least one red allele. This can occur in two scenarios: the plant could be homozygous, having two red alleles (one on each homologous chromosome), or it could be heterozygous, having one red allele and one white allele. In either case, the plant will display the red phenotype because the red allele is dominant.

Thus, stating that there is a red allele present on at least one of the two homologous chromosomes captures both scenarios and accurately reflects the conditions for a red-flowering phenotype in this species. This is why the selected answer points out that the red allele's presence is sufficient, as it encompasses both homozygous and heterozygous conditions leading to the red flower color.