Answer :
This is the case of complete dominance .
what is complete dominance ?
Complete dominance is a condition in which the effect of recessive allele is completely masked by the dominant allele .
In this the a1 and b1 act as a dominant allele , which add to the color and a2 and b2 are recessive one , which are completely masked by the other one . So it adds 0 unit to color .
Hence , a2 and b2 acts as recessive allele .
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