Answer:
Domiance
Explanation:
This is because the phenotype of vestigial wings is recessive.
Suppose the A gene controls the shape of the wings. If the allele "A" is present, they are normal. If the "a" allele is recessive, it will be vestigial.
Now an insect is diploid, so it has 2 copies of a gene. Then, a fly can be AA or Aa or aa.
AA and Aa have the dominant allele, so it will always be a normal winged individual. If it were aa it would have vestigial wings.
If you put to cross 2 flies Aa (heterozygous), the crossing will give the following offspring:
Aa X Aa = 1/4 AA + 1/2 Aa + 1/4aa
As you can see 1/4 or 25% of the offspring of two normal individuals can be vestigial.
The effect we see is known as dominance.
