In humans, the ability or inability to roll the tongue is a single gene trait. The allele for tongue rolling (R) is dominant to the allele for not being able to roll the tongue (r). Whether or not an individual can taste PTC is also an single gene trait. The allele for being a taster (T) is dominants to the allele for not being able to taste PTC (t).

Claudia cannot roll her tongue but can taste PTC. Her mom also can taste PTC but her dad is not a taster. Claudia's grandparents were all unable to roll tongues.

1) Based on the Info, what is Claudia's GENOTYPE?

I know she is "rr" because she cannot roll her tongue and it is recessive. However; I do not know the complete genotype for the PTC taster. I know it for sure is a "T," but is it "TT" or "Tt?" How do we know? Do we have enough info to figure it out?


1 Answer 1


Claudia's dad is not a taster, so he is tt. He passes one allele on to his daughter. Since he is homozygous, he can only pass t. Claudia is a taster, so she must have the dominant allele from her mother, who is also a taster. Thus, Claudia is Tt.

  • $\begingroup$ Thank you canadianer. That makes perfect sense. I have one additional question. What would the phenotypes be of Claudia's parents regarding tongue rolling? Because all the grandparents cannot does that mean both claudia's mom and dad cannot roll their tongues? $\endgroup$
    – Kyla
    Mar 23, 2015 at 3:55
  • $\begingroup$ @Kyla Yes, that's a logical conclusion. All grandparents are homozygous recessive which means they can only pass on the recessive allele to their children. Thus, Claudia's parents are also homozygous recessive. $\endgroup$
    – canadianer
    Mar 23, 2015 at 3:57

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