In pea plants, purple flowers are dominant to white flowers, and the purple and white alleles are represented with P and p, respectively. In a cross between two heterozygotes, what proportion of the offspring are expected to be purple?

75% of the plants are purple and 25% are white. The phenotypic ratio can be expressed as 3:1.

Explanation:

Heterozygous means that its genotype has two distinct alleles, i.e. a dominant and a recessive one. So the genotype of the plants is Pp, and each plant has two alleles since they are diploid organisms, which have two copies of each gene.

Each plant produces gametes, which are haploids cells. That is, they only have one copy of each gene (one allele). So, the gametes produced by Pp have a P or p genotype.  During fertilization, the maternal and paternal gamete are fused to form a diploid zygote where their genotype will have one allele from the father and one from the mother.

By making this monohybrid cross, we cross the gametes of each parent in the punnett square (see picture)

In the offspring, we see one PP genotype (homozygous dominant), two Pp genotypes (heterozygous) and one pp genotype (homozygous recessive). Since we know the P allele is dominant and it codes for purple color, a genotype only needs one P allele to express that phenotype. So Pp and PP organisms are purple, and only pp is white. That means 75% of the plants are purple and 25% are white. The phenotypic ratio can be expressed as 3:1.