How might a geneticist write alleles to show that a tall pea plant has one allele for tall stems and one allele for short stems?

T- tall stem
t- short stem
so, to represent what you asked for, it would be written Tt. Keep in mind, T is the dominant allele, and t is the recessive allele, in this example.

Ribonuclease activity is a type of nuclease that catalyzes the degradation of rna into small components.