for harebell plants, dominant alleles for two separate genes are needed to produce blue petal color. suppose gene 1 has alleles p and p and gene 2 has alleles b and b. only plants with at least one p allele and at least one b allele will have blue petals; all other genotypes lead to white petals. for the cross ppbb x ppbb, what fraction of the offspring are expected to have white petals?