A null hypothesis is the model that an observed difference is due to chance alone and not due to a systematic cause.
The alternative hypothesis is the opposite of the null hypotesis; it is the model that the null hypothesis is false.
The Philadelphia chromosome is an abnormal chromosome sometimes found in leukemia patients; it is the result of a translocation between chromosome 9 and 22.
The law of large numbers is a law stating that a large number of items taken at random from a population will (on the average) have the population statistics.