Which of the following resulted from codominance? 1)a person with ab blood type 2)a flower with pink petals 3)a rabbit with white fur 4)a mouse with brown fur

1) As blood groups A and B are equally dominant so the individual will express both characteristics in the phenotype. 
Notice how in all the 3 other examples the organism is only expensive 1 version (allele) of each gene. For example the mouse with brown fur may have 2 brown fur alleles (and so would have to express this characteristic even if it was recessive) but it could have 1 allele which is less dominant than the brown, in which case brown would be expressed. In this case the alleles were not co-dominant.

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