Dwarfism is indeed a genetic condition. Dwarfs are 4'10" or shorter, but the average height of an adult with dwarfism is 4 feet (so range is 2'8" to 4'8" or so, on average). Note that a dwarf will have disproportion of body parts. A midget is a term sometimes used to describe a little person that is proportional (so a very small size, but normal proportions). Note this term is generally considered derogatory and offensive, so it's not typically used. The less-offensive or more general way to describe either a dwarf or a midget is "little person", "little people", or "LP". More details here: http://www.lpaonline.org/faq-

Thank you Daniel, is there any other nice way to tell that someone is a dwarf?

Runt is usually only used to refer to the smallest animal in a litter, such as the smallest puppy or the smallest piglet. Normally a human would not be called a runt. Dwarf is a very short human, though they usually don't like to be called that.