Suppose "A" is a dominant gene for the ability to curl the tongue lengthwise, and "a" is a recessive gene for inability to do so. Which of the following couples can be certain that all their children will be able to curl their tongue lengthwise?

Suppose "A" is a dominant gene for the ability to curl the tongue lengthwise, and "a" is a recessive gene for inability to do so. Which of the following couples can be certain that all their children will be able to curl their tongue lengthwise?

a. father aa, mother AA
b. father Aa, mother Aa
c. father aa, mother aa
d. father Aa, mother aa