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Inheritance / Genetics Question - WJEC BY5 Watch

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    I'm pretty confused by Q2 b(iii) from this paper http://pastpapers.download.wjec.co.uk/s15-1075-01.pdf

    I put homozygous black as the answer (i.e. BR BR), however, this is incorrect and I can't see why.

    This is from the examiner's report:

    "‘Homozygous black’ was a popular incorrect answer but although these will always give black pups the genotype cannot be guaranteed from the phenotype."

    Why can't the genotype be proved? Surely if BR BR is crossed with BR BR, all the pups will also be BR BR?

    Thanks for any help
 
 
 
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