Final answer:
An organism with a genotype Rr showing phenotype R is an example of dominance, where the dominant allele R masks the expression of the recessive allele r. Dominance is a fundamental principle of genetics discovered through Mendel's hybridization experiments, illustrating how dominant alleles can determine an organism's phenotype.
Step-by-step explanation:
An organism with the genotype Rr expressing the phenotype for the dominant allele R is an example of dominance. This is because the presence of a dominant allele (R) masks the expression of the recessive allele (r) in the phenotype. Genetic concepts such as homozygous and heterozygous genotypes relate to the alleles an individual possesses.
A genotype like Rr, with two different alleles, is heterozygous. When the physical traits of heterozygous and homozygous dominant individuals appear the same, it is due to the law of dominance. This concept, first described through Mendel's hybridization experiments, explains why certain alleles (dominant) can mask the presence of others (recessive) when determining an organism's phenotype.
The phenotype of an organism is the set of observable characteristics, while the genotype is the genetic makeup which includes both expressed and non-expressed alleles. Mendel's experiments show us that despite having a different underlying genotype, heterozygous individuals (such as F1 hybrids) can display the same phenotype as a homozygous parent due to the presence of a dominant allele. This principle is witnessed in Mendelian traits, which exhibit a predictable dominant-recessive inheritance pattern.