In humans, the ability to roll the tongue is a dominant trait. The inability to roll the tongue is a recessive trait. If two individuals that are homozygous recessive for this trait have a child, what is the chance that the child will be able to roll his tongue?

0

Explanation:

This is a cross involving a single gene coding for tongue rolling in humans. The allele for tongue rolling (T) is dominant over the allele responsible for inability to roll the tongue (t).

Thus, a homozygous recessive individual will have a tt genotype and be phenotypically unable to roll his/her tongue. If two homozygous recessive individuals (tt) are crossed, all the offsprings will have a homozygous recessive (tt) genotype and be unable to roll their tongues.

Hence, the chance of two homozygous recessive individuals producing an offspring that can roll his/her tongue is zero (0) because it cannot occur.

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