The height of a certain plant is determined by a dominant allele T corresponding to tall plants, and a recessive allele t corresponding to short (or
dwarf) plants. If both parent plants have genotype Tt, compute the probability that the offspring plants will be tall. Hint: Draw a Punnett square.
(Enter your probability as a fraction.)

The characteristic "height" of a plant is determined by the alleles "tall" T (dominant) and "short" a (recessive). If both parents are Tt, you have to calculate the probability of the offspring being tall (TT or Tt)

To construct the Punnet square you have to make a table, where the parental alleles will be in the margins, for example: the father's alleles in the columns and the mother's alleles in the rows.

Each parent will produce a haploid gamete that will carry one of the alleles, so the probability for the offspring receiving one of the alleles is 1/2

Father (Tt): gametes will carry either the dominant allele T or the recessive allele t with equal probability 1/2

Mother (Tt): gametes will also carry either the dominant allele T or the recessive allele t with equal probability 1/2

Then you have to cross each allele to determine all possible outcomes for the offsprings. For each cell, the probability of obtaining both alleles will be the product of the probability of each allele (See attachment)

First combination, the offspring will receive one dominant allele from his father and one dominant allele from his mother: TT, the probability of obtaining an offspring with this genotype will be P(T) * P(T) = 1/2*1/2=1/4

Second combination, the offspring will receive the recessive allele from the father and the dominant allele from the mother, then its genotype till be tT with probability: P(t)*P(T)= 1/2*1/2=1/4

Third combination, the offspring will receive one dominant allele from his father and one recessive allele from his mother, the resulting genotype will be Tt with probability: P(T)*P(t)= 1/2*1/2=1/4

Combination, the offspring will receive both recessive alleles from his parents, the resulting genotype will be tt with probability: P(t)*P(t)= 1/2*1/2=1/4

So there are three possible genotypes for the next generation:

TT with probability P(TT)= 1/4

Tt with probability: P(Tt)+P(tT)=1/4+1/4=1/2⇒ This genotype is observed twice so you have to add them.

tt with probability P(tt)= 1/4

Assuming this genotype shows complete dominance, you'll observe the characteristic "Tall" in individuals that carry the dominant allele "T", i.e. individuals with genotype "TT" and "Tt"

So the probability of the plants being tall is equal to P(TT) + P(Tt)= 1/4+1/2=3/4

I hope this helps!