If both parents are true breeding, they have two of the same allele and are homozygous.
Let's use S for the dominant allele and s for the recessive allele. We will end up with the following combinations:
![\left[\begin{array}{ccc}&S&S\\s&Ss&Ss\\s&Ss&Ss\end{array}\right]](https://img.qammunity.org/2019/formulas/biology/middle-school/56zemud1o3yzzstaq8h264xkh05wdfmyh5.png)
As you can see from the cross above, all of the offspring will be heterozygous for this allele. Therefore, all of the offspring will display the dominant phenotype (straight-winged).