Homozygous dominant creatures mate. Note how each parent segregates their alleles to make their gametes. 

Because each parent is homozygous, only one type of gamete is produced. 

A homozygous dominant creature mates with a homozygous recessive creature. 

2 homozygous recessive creatures mate.

Note how the heterozygous father produced two different types of gamete. 

Note how the heterozygous father produced two different types of gamete.

Note how both heterozygous parents each produce 2 different kinds of gametes.