The dominant allele is T (it is always represented with the uppercase letter) and it codes for tall. This type of allele masks the effect of another allele.
The recessive allele t (it is represented with a lowercase letter) codes for short. This type off allele has its effect masked by a dominant allele.
When we look at the Punnett square we can see the offspring possible combinations:
TT: 25%
Tt: 50%
tt: 25%
The TT combination results in a tall offspring as the T allele is dominant.
The Tt combination results in tall offspring as T allele is dominant against t allele.
The tt combination results in short as both alleles ar ressesive and code for short.
As a result a 75% of the offspring will be tall and 25% will be short.
The answer is, 25%
