In reality its a boleean

so bit is fine just have to decide

0 = Male, 1 = Female
or 1 = Male, 0 = Female

How bout

Char(1)
M (Male)
F (Female)
U (Unknown)
B (Both)

i typically use a char(1) with a check constraint to allow only two values:

ie ALTER TABLE BLAH ADD GENDER CHAR(1) NULL CHECK (GENDER='M' OR GENDER='F')

varchar(1)

Null is unknown. That is the purpose of null.

>How bout

>Char(1)
>M (Male)
>F (Female)
>U (Unknown)
>B (Both)