Expert: Dana Krempels, Ph.D. - 8/20/2007

Question
Explain in detail how you would prove that a black guinea pig is heterozygous or homozygous
(Worth 11 points, have no idea how to get all 1 points!!)


Answer
Dear Caspian,

Before you can answer this question, you need to know whether the allele coding for the black fur is dominant or recessive to the other alternatives.  We'll assume for now that black is dominant (B) to whatever other color you're considering (b).

If you are assuming that the black condition is *dominant*, then the best way to determine whether the dominant-expressing guinea pig is BB or Bb is to mate him/her to a recessive-expressing guinea pig (bb).  You may have to get a lot of babies to know for sure, since guinea pig litters are not big!  But if you get even *one* baby that expresses the recessive condition, then you can confidently say that the black parent was Bb (unless a very unusual reversion mutation has taken place).

If many breedings of the two animals (B- x bb) produce only black babies, then there is a higher chance that the black individual is BB.  But you'd need a very large sample size to be absolutely sure of this.

I'm assuming here, of course, that you don't know the DNA sequences for the B and b alleles.  If you did, then you could take a small tissue sample and sequence the DNA to find out the genotype of your guinea pig.  But if you're using only Mendelian methods, then a test cross (B- x bb) is the way to go.

Hope that helps.

Dana