Expert: Walter Hintz - 12/8/2008

Question
In my biology class we were given the assignment to create a trihyrbid cross for the following problem;

In Guinea pigs, black hair (B) is dominant over white (b), rough coat texture (R) is dominant over smooth (r), and short hair (S) is dominant over long hair (s). Assuming these genes are on separate chromosomes, draw the Punnett square for a croos between a homozygous black, rough, short-haired Guinea pig and a white, smooth, long-haired one. What would the phenotypes of the offspring be? If two of the F1 offspring were crossed, draw a Punnett square for this cross. What would the genotype and phenotype ratios be for the F2 generation?

Now my question is, how do I set it up? And once it is set up and solved, how do I know what to use to solve the next part of the question?
Your help on this problem would be great.
Thanks.

Answer
Here is your F1 cross
 BBRRSS     x    bbrrss
    F1- BbRrSs
   Phenotype is Black ,rough and short

For the F2 you have to separate the chromosomes.  remember the separate during meiosis.


                        Eggs

sperms
      BRS      BRs     BrS     Brs   bRS    bRs    bRS   brs


 BRS  BBRRSS  BBRRss
 BRs
 BrS
 Brs
 bRS
 bRs
 bRS
 brs

I started to fill in the punnett square for you to finish
The phenotype ratio will be  9:3:3:1  Their are 8 phenotypes.
 For example
  BRS is black rough and short
  BRs is black rough and long
    Etc
 Finding genotype ratios is cumbersome.  You have to count all the different combinations. It is pointless

 Let me know if you have trouble