Expert: Dana Krempels, Ph.D. - 1/6/2010

Question
QUESTION: Hi!
If I have two plants with the genotypes Rrtt and rrTt where red color(R) is dominant over white(r) and being tall(T) is dominant over being short(t).If I complete a Punnet square, will it look loke this:
  R     r          r     r
t  Rt    rt          T  rT    rT
t  rt    rt          t  rt    rt
How can I diagram a cross between these plants(this is dihybrid cross)with sixteen boxes?What could be the possible genotypes and phenotypes from this cross?
Thanks a lot for helping!

ANSWER: Dear Sara,

I can't do your homework problem for you, but I can give you a hint.  Be sure that in your Punnett Square that each gamete has one of *each* gene.  For example, the Rrtt individual can produce gametes that are either Rr or rt.  You have shown only one gene on each side of the box, and you need to have *both* represented on both sides of the box.  The rrTt individual can produce gametes of either the rT or rt genotype.  

With that information, you should be able to set up your Punnett square.

Good luck!

Dana

---------- FOLLOW-UP ----------

QUESTION: Dear Dr.Krempels,
Was my first crossing correct? { the one with    R r      r r}
         t        T
         t        t  
I searched my entire textbook , but it does not contain any examples of a punnet square with 16 boxes. Could you show me an example? Or just the one or two of the crosses?
Thanks!

     Sara

Answer
Dear Sarah,

Your Punnett square won't have 16 squares because you're not doing a dihybrid cross.  Your parental organisms are homozygous for one of their traits each.  So line up the square like this:

Horizontal line parent:  Rt and rt only (Sorry about they typo in my first response!  Just now saw it.)
Vertical line parent:  rT and rt only

So you Punnett Square will have only four squares, since each parent can produce only two different types of gametes:

Rt x rT (RrTt)
Rt x rt (Rrtt)
rt x rT (rrTt)
rt x rt (rrtt)

I can't put this in a square with this text program, but I hope this helps you do the problem and any similar ones.

Good luck!

Dana