Question

the ability to roll your tongue R is dominant to lack of this ability r. what is the genotype of a man who can roll his tongue if his father couldn't?

Answer 1

If the mans father could not roll his tongue, then the father has a genotype of rr.

Now if the man (the son) can roll his tongue this mean he has a genotype of either RR or Rr.

Because the father has a genotype of rr, that means the son has to have at least one allele of r.

This means the genotype of the man is Rr.

Answer 2

The trait is dominant, only one of the alleles have to express the ability to roll your tongue (R).

Since his father is not able to roll his tongue his gentype must be rr.

His mother must have had either (Rr) or (RR)

      R       R                       R        r

 r   Rr      Rr               r    Rr       rr

 r   Rr      Rr               r    Rr       rr

The man can roll his tongue so his genotype must be either Rr or RR

As we can see from the punnet sqare the only genotypes that give him the ability to roll his tongue is Rr.