You are studying the genes for coat colour in guineapigs. So far here is what you know: the gene for colour of the fur is the "B" gene, with the dominant allele, B, leading ro black fur and the recessive,b allele leading to orange fur. You discover a mutation in a new gene, which seems to be involve in the production of spots. You observe that some guinea pigs have solid fur colour and others have a spotted phenotype with large white patches. It is easy to make a purebreeding spotted guinea pigs. Spotted guinea pigs that you know are B/B, when interbred will always give rise to some solid black and some pure white guinea pigs as well as spotted ones. This is true of spotted orange guinea pigs too- they produce some solid white, solid orange and spotted orange offspring. You have named the gene in question SP, for spotted.

a. propose an explanation for how the SP gene affects colour. Your explanation must be sonsistent with all the data above.

b. If you cross two dihybrid guinea pigs, ie B/b; SP/sp, what is the expected genotypic and phenotypic ratio of the offspring?