Final answer:
A diploid organism with one purple allele and one white allele for betal color gene has a heterozygous genotype (Pp), with the purple trait being dominant and expressed in the phenotype.
Step-by-step explanation:
If a diploid organism carries one purple allele and one white allele for the betal color gene, their genotype would be described as heterozygous (Pp). In this scenario, the purple allele is dominant over the white allele. In genetics, dominant alleles are represented with a capital letter (P for purple), while recessive alleles are represented with a lowercase letter (p for white). When an organism is heterozygous for a trait, it carries two different alleles for a given gene. If these alleles are different, as in the genotype Pp, the dominant trait, which in this case is purple flower color, will be expressed in the organism's phenotype. On the other hand, the recessive trait, white flower color, will only be expressed phenotypically when both alleles are recessive (pp).
Applying this to one of Gregor Mendel's principles of inheritance, when crossing two heterozygous organisms (for instance, Bb x Bb), the resulting offspring will generally display a phenotypic ratio of 3:1. This means that 75% of the offspring will exhibit the dominant phenotype, in this case, purple flowers, and 25% will display the recessive phenotype, white flowers.