In humans tongue rolling(R)  is an bent little finger(B) are dominant overnon-rolling(r) and straight little finger(b). A hetrozygous tongue roller who is also hetrozygous for bent little finger...

In humans tongue rolling(R)  is an bent little finger(B) are dominant overnon-rolling(r) and straight little finger(b).

A hetrozygous tongue roller who is also hetrozygous for bent little finger marries a non-roller with straight little finger. What are the chances of them having a child who cannot roll their tongue and has a straight little finger. With punnet square.

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Since there are two genes, the punnett square is more complicated. 

Parents:

RrBb -> Heterozygous tongue roller and Heterozygous bent

rrbb -> non roller and straight

Each parent can produce gametes with different possible combinations. For RrBb, the possible pairs are RB, Rb, rB and rb. For rrbb, since all are recessive, the pairs would be rb, rb, rb and rb. You can apply the FOIL method in the determination of gametes. 

RrBb x rrbb 

           RB         Rb         rB       rb

rb      RrBb       Rrbb      rrBb    rrbb

rb      RrBb       Rrbb      rrBb    rrbb

rb      RrBb       Rrbb      rrBb    rrbb

rb      RrBb       Rrbb      rrBb    rrbb

Results:

4/16 = 25% RrBb

4/16 = 25% Rrbb

4/16 = 25% rrBb

4/16 = 25% rrbb

The chances of them having a child who cannot roll their tongue and has a straight little finger (rrbb) would be 25%.

Sources:

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