3.18c

Allele key:

• B-blue petals
• W-white petals

1. The blue petal plant has a genotype of BB-two alleles 
2. The White petal plat has a genotype of WW-two alleles
3. If we cross the blue petal plant with the white petal plant we get an orange petal plant. 
4. We have an unusual third phenotype which is orange. This is known as the co-dominant.  
5. In the codominance both alelles the B and the W contribute to the phenotype.