Heterozygous organisms show presence of two alternatives alleles for same trait. For plant height, a heterozygous tall plant, will show both the T (for tallness) and t (for shortness) alleles, as T allele is dominant over t allele. When a cross is made between heterozygous tall plants (Tt), there are 50% chances that a randomly selected allele will carry allele for shortness (t) as shown below:
Thus, "the probability that a randomly selected gamete will carry 't' allele in a cross between heterozygous tall pea plants, is 1/2."