In the datura plant, purple flower color is controlled by a dominant allele P. White flowers are found in plants homozygous for the recessive allele p. Suppose that a purple-flowered datura plant with an unknown genotype is self-fertilized and that its progeny are 28 purple-flowered plants and 10 white-flowered plants.

Use the results of the self-fertilization to determine the genotype of the original purple-flowered plant.

Key Concepts

Dominant and Recessive Alleles

In genetics, alleles are different forms of a gene. A dominant allele, represented by a capital letter (e.g., P), masks the effect of a recessive allele (e.g., p) when both are present. In the case of the datura plant, the presence of at least one dominant allele results in purple flowers, while only the homozygous recessive genotype (pp) produces white flowers.

Genotype and Phenotype

The genotype refers to the genetic makeup of an organism, while the phenotype is the observable physical trait resulting from the genotype. In this scenario, the purple-flowered datura plant can have either a homozygous dominant genotype (PP) or a heterozygous genotype (Pp), both resulting in the purple phenotype. Understanding this distinction is crucial for determining the original plant's genotype based on progeny ratios.

Punnett Square and Progeny Ratios

A Punnett square is a tool used to predict the genotypes of offspring from a genetic cross. In this case, the ratio of purple to white flowered progeny (28:10) can be simplified to approximately 7:2. This ratio suggests that the original purple-flowered plant was likely heterozygous (Pp), as a cross between two Pp plants would yield a 3:1 ratio of purple to white flowers, consistent with the observed progeny.