Having the ability to roll or curl the tongue up along both sides to form a tube is a dominant trait. Which diagram represents the inheritance of the lack of this ability?

Let R - can roll tongue; r - cannot roll tongue.

Lacking the ability can be inherited only when the parents are both heterozygous or both lack the trait. For example, using a Punnett square, Rr x Rr will give genotype 1 RR:2Rr:1rr and 3 can roll tongue:1 cannot roll tongue phenotypes. If both parents lack the ability, all progeny will also lack the trait. The alleles are assorted and segregated independently.