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.

Verified step by step guidance

1. Determine the phenotypes and genotypes involved: Purple flowers are controlled by the dominant allele P, and white flowers are controlled by the recessive allele p. A plant with the genotype PP or Pp will have purple flowers, while a plant with the genotype pp will have white flowers.
2. Analyze the progeny: The progeny consists of 28 purple-flowered plants and 10 white-flowered plants. This indicates a phenotypic ratio of approximately 3:1 (purple:white).
3. Recall the expected genotypic ratios from self-fertilization: If the original plant is heterozygous (Pp), self-fertilization would produce offspring in a 1:2:1 genotypic ratio (PP:Pp:pp), which corresponds to a 3:1 phenotypic ratio (purple:white). If the original plant were homozygous dominant (PP), all offspring would have purple flowers.
4. Compare the observed results to the expected ratios: The presence of white-flowered plants (pp) in the progeny suggests that the original plant cannot be homozygous dominant (PP), as this would not produce any white-flowered offspring. The observed 3:1 phenotypic ratio supports the hypothesis that the original plant is heterozygous (Pp).
5. Conclude the genotype of the original plant: Based on the observed phenotypic ratio and the principles of Mendelian inheritance, the genotype of the original purple-flowered plant is most likely heterozygous (Pp).

Key Concepts

Dominant and Recessive Alleles

In genetics, alleles are different forms of a gene that can exist at a specific locus on a chromosome. 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 this case, the purple flower color is determined by the dominant allele P, while the white flower color occurs only in plants that are homozygous recessive (pp).

Genotype and Phenotype

The genotype refers to the genetic makeup of an organism, specifically the alleles it possesses for a particular trait. The phenotype is the observable expression of that genotype, such as flower color in the datura plant. In this scenario, the purple-flowered plant could have a genotype of either homozygous dominant (PP) or heterozygous (Pp), while the white-flowered plants must have the genotype pp.

Punnett Square and Mendelian Inheritance

A Punnett square is a tool used to predict the genotypes of offspring from a genetic cross. According to Mendelian inheritance, the ratio of phenotypes in the offspring can reveal the genotype of the parent. In this case, the observed ratio of purple to white flowers (28:10) suggests a specific genotype for the purple-flowered parent, which can be analyzed using a Punnett square to determine whether it is PP or Pp.