The offspring will have one allele for straight wings and one allele for curly wings.
Step-by-step explanation:
Let W be straight wings (big letter for dominant allele)
Let w be curly wings (small letter for recessive allele)
Each fly has two alleles.
If a parent is true breeding, it only has one type of allele.
Parent 1 = WW (straight wings)
Parent 2 = ww (curly wings)
Cross the parents: WW x ww in a Punnett square (see photo below)
This is the F1 generation (generation produced by crossing the first parents).
There are only Ww offspring.
Therefore,
The offspring will have one allele for straight wings and one allele for curly wings.