here is my question:

In certain plants, tall is dominant to short. If a heterozygous plant is crossed with a homozygous tall plant, what is the probability that the offspring will be short?

My Solution:

Let T be the allele for the tall phenotype and t be the allele for the short phenotype.

Then the parental cross would be Tt x TT = 100% tall phenotype (1/2 Tt, 1/2 TT).

Hence the probability of having short offspring should be $\fbox{0}$.

However, the answer key for this question says the answer is $\fbox{$\frac{1}{2}$}$. How can this be??? It really bothers me when the answer key is potentially incorrect because it makes me very unsure, and I end up wasting time trying to figure out if the key is wrong or if I am. Can somebody please let me know if what I did was correct/incorrect??


  • 3
    $\begingroup$ What's the source? I'd suggest it is an incorrect answer key... or does the question say anything about dominance (if t is dominant to T then the answer would be 1/2). Edit: re-reading your question T is dominant to t thus the answer should be 0. $\endgroup$
    – rg255
    Commented Dec 1, 2014 at 11:15
  • 1
    $\begingroup$ Agree with @GriffinEvo. If you have restated the question accurately, the given answer must be wrong. $\endgroup$ Commented Dec 1, 2014 at 11:32

2 Answers 2


Assuming you have stated the question correctly, the answer key is incorrect for exactly the reasons you have given and your reasoning and consequent answer are correct.

I would double-check you have read the question correctly and then conclude the answer key is incorrect. It happens.


This is a test cross - a cross with a recessive pair of alleles and either a heterozygous or homozygous pair of dominant alleles( eg- Tt X tt or TT x tt respectively). These crosses are carried out to check the if genes are homozygous or not. In this the ratio is always 1:1. So don't worry. :) Be confident about your answer, because it is obvious that the answer cannot be half if you work it out using the Punnett square.


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