Liquid water is less dense than frozen water. (you can see that when ice stays above water in your cup.)
The reason for that is that when water freezes it’s molecules position further away from each other(because of the 4 hydrogen bonds and the honeycomb shape that ice gets). So the volume increase and density decreases. (Density=mass/volume).
Well, the cell membrane regulates what goes in and out of the cell, it all so helps to shape it along with the cell wall (animal cells do not have a cell wall.) So it’s like a guard for the cell.
This case happens when both alleles are dominant then they will be equally expressed in phenotype
I can give you an example to understand this topic such as blood type: in the type AB both A and B are codominant = 2 dominant alleles are expressed together