Genetic Analysis: Gene interactions A mutagenesis screen was carried out to find mutations in a chalcone synthase (CHS) gene in a plant with purple flowers. CHS is a enzyme required for the biosynthesis of flavonoids which are an important family of pigments in plants. The CHS enzyme converts a colourless pigment to a purple pigment in a single step. Many white flowered mutants were identified from the mutagenesis screen. Two of the mutants identified from the screen had reduced levels of the CHS enzyme. Despite the reduced levels of enzyme activity both these plants had purple flowers and the same appearance as wild-type plants. A cross was carried out between these two mutant lines. The F1 progeny from this cross had purple flowers. F1 plants were self pollinated and large F2 population was analysed for flower colour. It was found that there was a ratio of 15 purple flowered plants to 1 white flowered plant. The cross were repeated and additional F2 populations were examined. These analyses confirmed the original findings. Provide an explanation for the 15:1 ratio and fully justify your answer. You may find it helpful to think about the following questions: 1) What is the genotype of the F1 progeny from the cross? 2) What is the likely genotype of all the purple flowered plants in the F2 population? 3) What is the likely genotype of the white flowered plants in the F2? 4) How many CHS genes might have been identified in the mutagenesis 5) What is the evidence for this? There are likely to be two CHS genes that are not linked. The F1 progeny is heterozygous at both loci. In the F2 population the purple flowered plants have at least one copy of the wild-type gene for one of the CHS genes. A mutation in either one of the CHS genes reduces CHS activity overall, but this is still sufficient to synthesise enough purple pigment in the flowers. The 15:1 ratio is modified 9:3:3:1 ratio. The double mutant appears in 1 out 16 plants in the F2 generation. These plants carry homozygous mutations for both CHS genes. In the double mutant, the level of CHS activity is likely to be absent or very low. There is either not enough enzyme, or there is no enzyme available to generate the purple pigment. You could test this by measuring the level of CHS activity in the double mutant. This result is consistent with the single step conversion of a colourless pigment to purple pigment. At least two genes convert the colourless pigment to a purple pigment and act at the same point of the biochemical pathway.