This is just a basic project for fun and experience.
The shelf will measure about 48,24,12 (H,W,D).
Do you think this would hold an even load of 50 lbs per shelf with 5 shelves?
Each shelf is capable of holding the load that you are intending with shearing or sagging. Where this design fails is in racking, both front to back and side to side.
Traditional shelves are built with solid sides and a back. This serves to hold books, or items onto the shelves, but this really is only a secondary function.
Primarily, the solid sides and back resist racking forces.
In you design, you have replaced the solid sides with a narrow stile at the front and back, and only the horizontal rails at each shelve. What this does is reduces your resistance to racking down to only the 2x2 area of your lap joints on the sides, and on the back it is even worse, being reduced to the butt joints of your shelves rails where they meet the verticals.
I would first recomend increasing the width of your vertical stiles to at least a 1x3 or 1x4. This will increase the area of your lap joints, thus increasing the resistance to racking front to back.
Next I would consider adding a full back. If you don't want a full back, then I would add a vertical 1x4 on the left and right side of the back, just like you have on the sides.
The weakest part of the design is how your substantial load is being carried to the uprights. There's not that much you can do for those joints as drawn...
I'd suggest you add actual 1x2 members beside your existing uprights that will carry the weight of the frame (perimeter) of each shelf. It's easy to build this way, and will look a little less spindly. (Comment back if you need a drawing.)
Comments above about cross-bracing to prevent racking should be taken seriously, unless you're attaching securely to a wall.
edit to add a terrible picture. (Hope you get the idea, though.)
The red parts (done all the way down, of course) will transfer load from the shelves to the floor.
One more edit:
glue and mechanically fasten (screw or nail) the additional uprights for strength.
if you start from the ground and work your way up, all you have to do is cut the additional uprights exactly the same length and the unit almost builds itself. (Exact length is easy on a chopsaw where you can gang cut everything; a bit harder with a handsaw.)
a cheap and cheerful crossbrace can be found at your local Ikea: http://www.ikea.com/us/en/catalog/products/87749600/ ("OBSERVATÖR Cross-brace, galvanized", $5). This would keep the back of the unit from racking, and the sides would take care of themselves.
The sagulator calculates a sag of 0.03 in over 24" for a 50 lb uniform load on a single 1x2 of (Ponderosa) pine laying on its face. So your 4-board shelf should be stable given that weight load. A 12" 1x2 on its side can support in excess of 1000 lb, so no worries about the shelf supports breaking, either.
Note that, although each shelf could hold the same amount of weight, unless the unit is secured to the wall, loading it down that way could lead to it tipping over. Even if you do load the higher shelves lighter than the lower shelves, I would still secure it to the wall, especially if there are small children around.