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According to Fischbein and Schnarch, one misconception students have revolves around the concepts of sample space, sample size, compound events and simple events.  By not having an innate understanding of sample space, a student may falsely assign similar probabilities to two different events.  Consider two parents who are about to have a baby.  If the father is homozygous for brown eyes and the mother is homozygous for blue, will the probability that the child has blue eyes be equal to, less than, or greater than that of having brown eyes?  Most students will say that the child has an equal chance to have either color.  The Punnett Square below will give to the student the evidence needed to alleviate this misconception.

Misconception - The students assume that since both parents are homozygous for their eye color, than each will bring an equal weight to the color-of-eye decision making table.   Hence, they will assume a 50/50 outcome for eye color.  The lack of understanding of how, in this case, simple genetics works leads them to think that the possible outcomes or sample space is only: blue or brown.  

Clarification - There are four possibilities whereas most students assume only two.  The reality is that the possible outcomes are: Bb, Bb, Bb, and Bb!  All Brown!    

What does the student need to understand?

1.  Brown eye color is dominant over blue eye color.

2.  Genotypes are the genetic make-up of the individual.

3.  Phenotypes are the physical appearances of the genotypes.  

4.  All individual probabilities assigned to each outcome in the sample space must add up to a total of  1.