In most cells, DNA directs the production of a molecule of mRNA which then enters a ribosome. The ribosome produces the final product, a protein. Retroviruses do the opposite. They use the enzyme reverse transcriptase to use a template of mRNA to produce DNA.
Explanation:
The central dogma of molecular biology describes how a cell uses its genetic information to synthesize a protein. The first step called 'transcription' consists of using an RNA polymerase enzyme and a fragment of DNA (i.e., gene) as a template to synthesize an RNA molecule, usually a messenger RNA (mRNA). Subsequently, the genetic code in the mRNA sequence is read by the ribosomes in order to produce a protein, a process known as 'translation'. Retroviruses are viruses that have the ability to integrate into the host genome by using a reverse transcriptase enzyme that allows reverse transcription of mRNA to complementary DNA, which then integrates into the host's genome.
If the purple plant was homozygous dominate then it can only pass along a dominate allele to its offspring so despite the genotype of the other parent 100% of the offspring will reprsent a purple phenotype.