Markedness refers to expressions that can be their own hyponym and contrast with another hyponym. This often occurs for gender in animals:
Female cow = cow
Male cow = bull
Often the expression that is its own hyponym is the most common of the taxonomic sisters (i.e. you hear “cow” used more commonly than “bull”).
Markedness is not always this closely related to hyponymy; it can also be applied to antonyms. In the antonymic pair tall/short, tall is marked. We can see this from expressions like “how tall are you?” and “I’m 6 feet tall”; you would not expect to hear “I’m 6 feet short”.