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    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.

    Why is the answer D and not C?

    Although C seems right in terms of the ratios it could well be that although the ratio for say heterozygous is 1:1 the cross happens to produce only black mice. Therefore C is wrong and D is right.
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