Question

Let's suppose that there is a locus (denoted by P) that controls fruit coloration in a species of grapes. For a really long time, farmers have known that there is a recessive mutation in the P locus, such that PP and Ppplants have purple grapes, whereas homozygous recessive pp plants have yellow grapes. More recently, plants with yellow grapes started showing up in a northern CA farm. So you first decide to determine if the old and newer mutations causing yellow grapes actually happen in the same gene or not. To that end, you cross a classic pp plant with one of the new NorCal mutants, and you obtain 100% progeny with purple grapes. In genetics, we call this "complementation", and it indicates that the new mutation is also recessive but in a different locus (that we will call T). What happened...? Let's take a look - the classic pp plants must have had a wild type (normal) T locus (i.e., they were pp TT). On the other hand, the mutant plants were homozygous recessive for the T locus (tt), but must have been homozygous wild type for the P locus (i.e., they were PP tt). If you crossed pp TT to PP tt plants, you expect your F1 to be 100% heterozygous for both loci (Pp Tt). And they all have purple grapes, of course. Then, being a good Mendelian geneticist, you decide to self-cross some of these Pp Tt plants (i.e., you set up a dihybrid Pp Tt x Pp Tt cross). Perhaps not too surprisingly, you see that the phenotypic distribution of the F2 is 9 purple grapes : 7 yellow grapes. You decide to look a little deeper into this new T locus, and after several months of molecular biology in the lab, you conclude that the protein encoded by the T locus is a transcription factor. More specifically, T codes for a transcriptional activator with a binding site in the proximal promoter region of the P locus. In 150 words or fewer, explain the molecular basis of this epistasis. Essentially, you need to describe the gene expression and regulatory steps that explain why a homozygous recessive mutation in either locus would result in yellow grapes.

Answer 1

In epistasis effect of one gene in a particular locus is dependent on the presence of one or more genes i.e the phenotypic effect of one gene is masked by a different gene in a different locus. The above case is a classic example of positive epistasis. The transcriptional activator T is masking effect of homozygous recessive mutation pp and prevents formation of yellow color. Presence of t in otherwise pp plants produces yellow color. In P locus the P gene product causes purple pigment. But mutation in the P gene (i.e p allele form) probably causes inhibition of the pigment transcript production.Some gene product from p allele may act as repressor. When T binds to proximal promoter of P locus containing mutated p, the transcript is produced (means the repressor effect of the p allele gene product is removed). Transcription proceeds from promoter producing purple pigment.