Aheterozygous, three-horned alien from mercury has met and fallen in love with a four-horned alien from jupiter. what would happen if there was an intergalactic mating between the pair?
t is for three horns
f is for four.
for the three horned one i have tf
what would it be for the four horned one?

Because the three-horned alien is heterozygous, we know that three must be dominant to four, because the gene for the three horns is "hiding" the gene for four horns. Therefore, the three-horned alien has the genotype Tt (T for three horns, and t for four horns). The four horned alien must be tt, because that is the only way that a recessive trait may be seen.  If you solve the punnet square on a cross between Tt and tt, you end up with half three (heterozygous) and half four (homozygous recessive) it is a bit easier to explain with something a little "closer to home" if you want me to explain it again, just say so, I don't mind!