Question

what does a filled circle or square represent in a pedigree?
a completely healthy individual
a organism of similar species
a deceased individual
an individual affected by a genetic disorder

Explanation:

In pedigree analysis, symbols have specific meanings. A filled (shaded) circle (female) or square (male) represents an individual affected by the genetic disorder being studied. A completely healthy individual is typically an open symbol, a deceased individual may have a diagonal line, and similar species are not represented this way. So the correct option is the one about an individual affected by a genetic disorder.

Answer:

an individual affected by a genetic disorder (the fourth option: "an individual affected by a genetic disorder")