Genetics
One risk factor for asthma is a condition called atopy, which is essentially a predisposition to be allergic to things. (Atopy is believed to be present in around 20% to 40% of humans.) The evidence is quite clear that heredity is partially responsible for a person having atopy. In fact, progress has been made in recent years in identifying where on the chromosomes some of the genes responsible for atopy might be located.