Let us say that we symbolize the dominant allele by E (being yellow). So, in the first generation, since fruit flies reproduce sexually, we have that one allele will stem from the father and one allele from the mother. So, the result is invariably a fruit fly with genotype Ee. This means that its phenotype (what it looks like, the characteristics defined by genes) will be yellow since it contains a dominant allele. Thus, all fruit flies from this crossing will be yellow with a genotype of Ee (one dominant, one recessive allele).
