A certain breed of mouse can have fur that is either black or brown. A mouse breeder performs a test cross using a brown mouse to determine the unknown genotype of a black mouse. There are four offspring produced by the cross. What conclusions can be drawn?

A. If the black mouse is homozygous, one of the four offspring must be brown.
B. If the black mouse is heterozygous, three of the four offspring must be black.
C. The black mouse must be homozygous if all four offspring are black.
D. The black mouse must be heterozygous if any of the offspring are brown.

D

Question 13 was the least effective on the paper in terms of discriminating between the candidates, which is unusual for a genetics question. Teachers raised a variety of objections to it. The statistics suggest the problem was that some good candidates chose C rather than D. To answer the question correctly it was first necessary to read the second sentence carefully and deduce from it that brown mice are homozygous recessive because they were used in the test cross. Answers A and B could then easily be eliminated. Answers C and D needed to be carefully considered. C should have been rejected because four offspring are too few to be sure that the black mouse was homozygous. There is a one in sixteen chance of getting four black offspring from a cross between a heterozygous black mouse and a homozygous brown recessive. On the other hand, answer D is undoubtedly correct. To cope with this type of question, candidates need to understand the difference between the possible outcomes of a cross as shown in a Punnett square and actual outcomes, which cannot be expected to correspond exactly with Mendelian ratios.