Two tall parents can have a phenotypically short child if both parents carry a recessive gene for shortness, even though they themselves are tall, and the child inherits two copies of that recessive gene, one from each parent. 

While genetics play a significant role in determining height, it's not controlled by a single gene, but rather by multiple genes and their interactions. If we use the letters "T" for tall and "t" for short, the parents would be "Tt". The child could have TT (tall), Tt (tall), tT (tall) or tt (short). 

Kindly note that although genetics are a major factor, other factors like nutrition, hormones, and medical conditions can also influence a child's height.