Answer :
Considering the height trait, being T tall, and t short, the reason why the offspring of the breed between YyTt and YyTT will always have a T allele, as we can see in the following Punnet square:

Considering the height trait, being T tall, and t short, the reason why the offspring of the breed between YyTt and YyTT will always have a T allele, as we can see in the following Punnet square: