Why are most genes inhibited in an average animal cell

Most genes must be turned off because cells play specific functions, thereby only a limited number of genes must be expressed in each cell type in order to produce proteins required in the cell  

Explanation:

Gener regulation is a critical molecular mechanism required for normal cell functioning. In consequence, genes are turned on and off depending on the cell type. For example, in the brain, cells that will be differentiated into neurons will be expressed, while genes involved in the differentiation of undifferentiated myoblasts into muscle cells will be turned off. Thus, genes are switched off in order to produce only those specific proteins which are required for each cell during specific developmental programs and functions carried out by each cell type.

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!