The correct answer is gray bodied, normal-winged flies plus black bodies, vestigial-winged flies = 17%.
In the given case, the genes are linked, thus, there is no independent assortment. B is gray body (dominant), b is black body (recessive), W is normal wings (dominant), and w is vestigial wings (recessive).
The parent generation would be: bbWW × BBww
After crossing the parent generation, the F₁ generation would be BbWw
Even so, during forming gametes for the F₂ generation, one basically cannot arrange or randomly assort the alleles. One has to take into consideration that the chromosomes are Bw and bW, thus, they will produce crossovers.
25% Bw-Bw, 50% Bw-bW, 25% bW-bW
As there is a crossover event, so one cannot get an F₂ who was BW (gray bodied and normal winged), or bw (black bodies and vestigial winged). Hence, the mentioned answer indicates or suggests towards all the order of instance or occurrence where a crossover event has occurred that associates back to the map units.
