The offspring have yellow body colors, but carry the blue gene.
This must mean the genotype Yy is the only possibility.
The uppercase Y to reflect the dominant yellow body color (phenotype) and the lowercase y to reflect the recessive blue gene being carried.
This must mean Yy is the genotype of one parent, while the other parent will have genotype yy. We do not go for Yy in the 2nd parent because then that would give one offspring as YY and another offspring as yy.
Check out the Punnett Square shown below.