The Chocolate gene (Choc+) is a sex-linked gene. Males need two copies to express it, whereas females only need one copy. Females can only pass their copy of (Choc+) to their male offspring.

Breeding a chocolate male over a black female will result in 50% chocolate females and 50% black split males. 

Split males only carry one copy of (Choc+). Breeding these back to a chocolate female will result in 25% black females, 25% chocolate females, 25% black males, and 25% chocolate males.