Which one of the following genotypes is homozygous dominant and which one homozygous recessive in regard to tongue rolling: Rr, IT, RR

Genotype is a genetic expression of a character in terms of alleles. In case of homozygous condition, both the genes controlling that character are identical. The gene which expresses itself in heterozygous condition is called a dominant gene. Thus, the genotype of homozygous dominant is RR.

While the gene which fails to express itself in heterozygous condition is called a recessive gene. Thus, the genotype of homozygous recessive is rr.