Final answer:
There are four unique tetrapeptides that can be formed from three leucine and one threonine, since the threonine can be placed in any of the four positions, with the leucines filling the other positions.
Step-by-step explanation:
When forming tetrapeptides from three leucine and one threonine, we must consider the different permutations of these amino acids. Since tetrapeptides are formed by linking four amino acids through peptide bonds, the order of amino acids affects the final structure.
The given combination where we have three of one kind (leucine) and one of another (threonine) leads to limited permutations. In this case, the threonine can occupy any one of the four positions, and the three leucines will fill the remaining spots.
This is similar to arranging four objects where three are identical, and one is unique, which can be done in 4! / 3! = 4 ways (since there are 3! ways to arrange the three leucines amongst themselves, which does not change the overall sequence). Therefore, four unique tetrapeptides can be formed from this set of amino acids.