If a person cannot roll their tongue they posses two recessive genes for this characteristic. If neither of your parents can roll their tongue, then you will not be able to either. Neither would have possessed the dominant gene to pass on to you. The notation often used to describe the combination of genes denoting a characteristic is a capital letter for the dominant gene and a lower case letter for the recessive. A mutation is a change in the DNA of an organism.
Mutations occur randomly, that is, any gene can undergo mutation at any time.
It is possible for an accident to happen during meiosis where an extra chromosome reaches the gamete. This is an example of a mutation which affects a whole chromosome. Mongols have 47 chromosomes instead of the usual 46. As the alteration in the gene can vary it will be slightly different for each family. Once this broken gene is present it becomes hereditary.