Question

True breeding red-eyed Drosophila flies with plain thoraxes were crossed with pink-eyed flies with striped thoraxes.
Red eye plain thoraxes × Pink eye stripped thoraxes
The ${F_1}$ flies were then crossed against the double recessive.
The following ${F_2}$ generation resulted from the cross:

${80}$	${16}$	${12}$	${92}$
Red eye	Red eye	Pink eye	Pink eye
Plain thoraxes	Stripped thoraxes	Plain thoraxes	Stripped thoraxes

What percentage number of recombinants resulted from the test cross?
A. ${12}$
B. ${14}$
C. ${16}$
D. ${28}$

Answer 1

Hint: The ${F_2}$ generation helps determine the number of recombinants from test cross in percentage. The total number of recombinants and total number of offsprings in ${F_2}$ generation shall be taken into consideration.

Complete answer:
Red eyed flies having plain thoraxes and pink eyes flies having stripped thoraxes are the non-recombinant organisms as they have their own traits respectively. In these two, there was no mixing of the genetic traits. After crossing the two varieties, the ${F_1}$ progenies were further crossed against double recessive to obtain flies with four respective traits including red eyed flies having plain thoraxes, red eyed flies having stripped thoraxes, pink eyed flies having plain thoraxes, and pink eyed flies having stripped thoraxes. The red eyed flies with striped thoraxes and pink eyed flies with plain thoraxes are the recombinants as mixing of genes lead to expression of traits that were not present earlier among those flies, respectively.
Firstly, the total number of flies in ${F_2}$ generation is calculated by simply adding all together. Furthermore, the total number of recombinants is calculated by adding the numbers of flies with recombinant traits. The calculation is then done to determine the percent number of recombinants in the second generation resulting from the test cross.
\[
  {\text{Red eye plain thoraxes}} = 80 \\
  {\text{Red eye stripped thoraxes}} = 16 \\
  {\text{Pink eye plain thoraxes}} = 12 \\
  {\text{Pink eye stripped thoraxes}} = 92 \\
  {\text{Therefore, total number of offsprings (N)}} = 80 + 16 + 12 + 92 \\
   = 200 \\
  {\text{Number of recombinants}} = {\text{Red eye stripped thoraxes}} + {\text{Pink eye plain thoraxes}} \\
   = 16 + 12 \\
   = 28 \\
  {\text{Number of recombinants}}\left( {{\text{in percent}}} \right) = \dfrac{{{\text{Number of recombinants}}}}{{\text{N}}} \times 100\% \\
   = \dfrac{{28}}{{200}} \times 100\% \\
   = 0.14 \times 100\% \\
   = 14\% \\
 \]

The correct answer is option B, stating ${14}$.

Note:
Recombinants are often referred to those organisms which express a new trait due to its crossing with another organism having that particular trait. If it fails to acquire a significant part of DNA responsible for expression of the trait, non-recombination is then marked with expression of original traits and as a result, no intermixing took place.