You Have Been Studying the Effect of Loss-Of-Function Mutations in the Cdk

You have been studying the effect of loss-of-function mutations in the Cdk inhibitor protein (CKI) p21. You add the drug fucoxanthin to cell cultures harboring either wild-type or mutant versions of the p21 gene. Fucoxanthin is known to induce cell-cycle arrest in G1. After a day, you add the thymidine analog BrdU to the culture media, collect the cells after an hour, treat them with anti-BrdU antibody and the fluorescent DNA stain DAPI, and finally subject them to fluorescence-activated cell sorting (FACS). The FACS data can be viewed as a two-dimensional dot plot composed of thousands of dots, in which each cell is represented by one dot at coordinates that correspond to the intensities of the DAPI fluorescence signal (X axis) and BrdU fluorescence signal (Y axis) for that cell. Answer the following question(s) according to the simplified dot plot below, generated from your experiment.

-Which one of the FACS plots (a or b) would you expect to correspond to the loss-of-function p21 mutants? Write down a or b as your answer.