What are the genotypes and phenotypes of a dihybrid cross?

_____DY_____Dy_____dY____dy
DY    DDYY     DDYy     DdYY     DdYy
Dy     DDYy     DDyy      DdYy     Ddyy
DY    DDYY     DDYy     DdYY     DdYy
Dy     DDYy     DDyy     DdYy     Ddyy

u can take half the chromosomes from one parent and one half from the other parent. so 2 chromosomes from each parent since there are 4 (either DdYy or DDYy). so u find the possibilities for each parent. parent 1(DdYy) can give a D and a Y or a D and a y or a d and a Y or a d and a y. so u write those at the top or the side of the punnet square(it doesnt matter) u do the same thing for parent 2. DY or Dy or DY or Dy (write them all)

then just cross the different possibilities and u hav a punnet square. the genotypes are the DdYy formula whereas the phenotypes are what the offspring look like. (a DDYy phenotype would be the same as a DDYY phenotype since it shows the dominant traits for both. however the genotypes would b different since YY=/= Yy)

Its quite simple.
For Genotype find the combination of chromosomes possible with each trait. a.for DdYy you can have (choose one from D/d and other fromY/y) DY,Dy,dY.dy, similarly for DDYy you can have DY,Dy,DY,Dy(which essentially here is DY,Dy, a repetition, so take only DY,Dy)

Now draw a box(punnet square) with rows and columns and distribute these evenly on rows and columns. you get
                   ....DY:....   Dy:....    dY:..    dy:..
_____________________________ _________________
DY:DDYYDDYyDdYYDdYy
_____________________________ __________________
Dy:DDYyDdyyDdYyDdyy
_____________________________ ____________________
DY:DDYYDDYyDdYYDdYy
_____________________________ ___________________
Dy:DDYyDdyyDdYyDdyy

Now, phenotype you can decide simply by considering the character will be dominant even if single dominating gene(capital alphabet) is there but recessive only if both the genes are recessive genes(small alphabet).