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.