Expert: Dana Krempels, Ph.D. - 12/7/2008

Question
Hi!
I am studying for my final and am confused about how to perform two and three point crosses.  I do not get how you can figure out the maternal genotype if given the parental genotype.  How do you also get the genotypes of the F1 progeny?  I can tell which are the parental and the recombinants if the numbers are given to me, but otherwise I am confused.

Thank you very much,
Charleen

Answer
Dear Charleen,

This is far too complicated for a single email, and really is in the realm of "please come sit in on my genetics lecture, and I'll try to clear things up for you."  Linkage and chromosome mapping are one of the topics in genetics that students seem to find most confusing, and trying to show you in an email will be pretty much impossible. I'd strongly urge you to go to your instructor's office hours for help.  That's what s/he's there for!  :)

Your sentence:  "I do not get how you can figure out the maternal genotype if given the parental genotype."  doesn't make sense to me.  A maternal genotype *is* a parental genotype.  Do you mean how to figure out the *paternal* genotype if you know the maternal and F1 genotypes?  That should not be too hard to figure out, as it's like supplying the missing integers in an algebra equation. :)

You can't get the genotypes of the F1 progeny without knowing both parental genotypes any more than you can get the answer to an equation if you are missing some of the equation.  So again, I can't help you there without more information.  But I'm assuming you know how to do a Punnett square.

I hope that if I refer you to my lecture notes on linkage and mapping that they might help you understand this complex topic.  There are examples that you can walk through.  Please find them here:

http://www.bio.miami.edu/dana/250/25008_6.html

I hope that will help.  Good luck on your finals!

Dana