What observation led a geneticist to hypothesize that short wings in fruit flies are a recessive trait?

The observation that led a geneticist to hypothesize that short wings in fruit flies are a recessive trait is that flies with long wings can produce offspring with short wings. This is crucial because if long wings were a dominant trait, one would not expect a long-winged parent to produce any short-winged offspring unless the long-winged parent also carried a recessive allele for short wings. This indicates that short wings must be the result of a recessive allele, as the presence of a dominant allele (for long wings) can mask the expression of the recessive trait in the parents, yet still can be passed on to the offspring.

In genetics, the behavior of traits across generations, such as the mixing of alleles from parents, plays a key role in determining if a trait is dominant or recessive. The ability of long-winged flies to produce short-winged offspring suggests that both parents must carry the allele for short wings, which supports the idea of it being recessive. Other observations, such as the survival of flies with short wings or their prevalence in the wild, do not directly provide insights into the inheritance patterns necessary to classify traits as dominant or recessive.