How do you write the heterozygous dominant genotype?

If "T" is for the dominant allele of a trait (eg. phenotype of brown fur) and "t" is the recessive allele of a trait (eg. phenotype of white fur), then a heterozygous dominant genotype for this trait would be " Tt ".

Heterozygous means both alleles are different (so Tt, not tt or TT)

A genotype is what the genes say (one allele for brown fur, one allele for white fur), while a phenotype is what the organism displays (in this case, brown fur, because the allele is dominant and therefore only needs one allele present for this trait to be displayed)