Crosses of pure-breeding lines can result in progeny phenotypes that don't appear to follow mendel's rules. explanations for this include:
a) no definitively dominant or recessive allele
b) more than two alleles exist
c) multiple genes involved
d) gene-environment interactions

b) More than two alleles exist

Explanation:

There are cases where more than two alleles determine a character in the population. This type of inheritance is called multiple alleles (or polyalelia). Classical examples of polyalelia are hair color determination in rabbits and blood groups of the ABO system in humans.  

In the attached figure we can see another example of polyalelia where a flower is self-fertilized generating different phenotypes in its offspring.

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d. making up measurements to record.