Use a punnett square to explain how a dominant allele masks the presence of a recessive allele

 For the answer to the question above, 
DD <-- all (homozygous) dominant 
dd <--- all (homozygous) recessive 

DD-X dd 

__d__d 
D Dd Dd 
D Dd Dd 

^your Punnett square 

since there is a dominant gene in Dd, Dd's phenotype will be of the dominant. Therefore, an all dominant allele X an all recessive allele will yield an all dominant phenotype offspring.

Cotton was the very first