Phenotypic ratio of linked genes in a dihybrid cross?

3:1.


Gametes - PR and pr x PR and pr gives PPRR,PpRr,PpRr and pprr

The exact ratio depends on how tightly linked they are and how often recombination occurs. However the general pattern is that you get two classes of progeny that are like their parents and are the most common and then two recombinant classes which have much smaller numbers.

If for example P and R genes were linked 10 map units apart, you would expect to get 90% parental classes and 10%  recombinant classes.

However, if you cross two heterozygotes it may be difficult to work out linkage properly, as some of the recombinants may be hidden in the parental classes - so you need to do a test cross with one double homozygote recessive to identify each parental and recombinant class individually.

This last bit is hard to explain without a drawing. If your parents were Pr/pR then the non-recombinant gametes would be Pr and pR, while your recombinant ones would be PR and pr, if you were to sit down and work out the progeny from these different gametes you would see what I mean.