What does the notation Rr mean to geneticists?

R: dominant allele

r:recessive allele

Heterozygous: two different alleles

Homozygous: two of the same allele

Rr is therefore heterozygous (A) because there is 1 dominant and 1 recessive allele - - > two different alleles

This notation provides quite a bit of information, to geneticists, this notation indicates that the set of alleles, which are 2 are found on the person's or organism's autosomal chromosome and not on its sex chromosomes, for humans it is X and Y. Now depending on the mode of inheritance, this allelic combination for whatever phenotypic or other trait that it implies, will be different.

For instance if it is incomplete dominance, it would showcase an intermediate between the 2 pure breeding parental organisms. Assuming it is complete dominance, this genotypic combination is heterozygous, as it contains one dominant and one recessive allele for the trait that it codes for.