Expert: Dana Krempels, Ph.D. - 7/7/2007

Question
I am having trouble setting up a punnet square for two traits.  One being white or black hair, and short and long hair.  The thing is I don't know which trait is dominant.

Answer
Dear Garrett,

To set up a Punnett Square for two traits, you don't really need to know which trait is dominant and which is recessive.  You can simply designate the two alleles with different symbols.  For example, white hair could be B and black hair could be B'.  Long hair could be S and short hair could be S'.

Remember that if you're crossing two individuals heterozygous for both traits that the alleles will segregate randomly into the gametes, so that each individual could produce the following combinations:

BS
BS'
B'S
B'S'

All you need to do is set up a 16-square matrix with those gamete genotypes on the sides, and you should be able to figure out the possible offspring genotypes without any problem.

To determine the phenotype, however, you will need to know which trait is dominant and which is recessive.  The book giving you the problem should tell you that.

Good luck,

Dana