You might check out the sum function in FileMaker help...
Sum ( Field ) gives exactly the same result as just referring to Field.
Sum adds up the values from a "list of fields" supplied to it as parameter(s).
You can compute a sum such as: Sum ( field1 ; field2 ; field3 ; etc...)
Or Sum ( repeatingField ) //sums up repetitions
Or Sum (relatedtable::field ) , where it sums up the contents of field for all records related to the current record of the table where you defined the sum.
From the syntax, it appears that ethnicity_gender_marker is defined in the same table as where you are computing this sum. If so, you might define a calculation field as If ( ethnicity_gender_marker = "C" ; 1; 0 ) set to return "number" as its return type and then define a summary field as the "total of" this calculation field. From a related table, you also have the option of computing this same total from the sum function.