Give below answer as per your knowledge .(Sample sources :
http://www.sciencedirect.com/science/article/pii/S0092867400812246​)

1.You have identified a novel organism that has only four cell
types (neural, muscle, gut, and germ cells) and twenty-one cells in
the adult. You sequence its entire genome and to your overwhelming
surprise discover that after you have accounted for all of the
housekeeping genes (genes necessary to keep cells alive which are
expressed in each and every cell) and all of the terminal
differentiation genes (genes required for the final phenotype of an
adult cell, for example, the sodium channel in neural cells), there
are only twenty five (25) genes left. Propose how these twenty
genes can give rise to this novel organism from the one cell stage
through cell type differentiation in the adult. You should sketch
your organism at each cell division (you will need at least five
“waves” of synchronous division to get to 21 cells) showing which
genes are expressed in each of the cells (and which genes they
regulate to produce the phenotype); drawing the promoter/enhancer
region of selected genes would be an effective way of showing gene
regulation. Please be sure to explain your proposed logic for the
development of your organism. (You may arrange the cells in your
organism in any way you choose). You can use real genes or genes
that you “make up” – but be sure to explain their function (e.g.
transcription factor) and ground them in reality.