One possible DTD is:

<!DOCTYPE id_data [
    <!ELEMENT id_data (name, date_of_birth, blood_group?)>
    <!ELEMENT name (firstname, middlename*, lastname)>
    <!ELEMENT firstname (#PCDATA)>
    <!ELEMENT middlename (#PCDATA)>
    <!ELEMENT lastname (#PCDATA)>
    <!ATTLIST date_of_birth day CDATA #REQUIRED>
    <!ATTLIST date_of_birth month CDATA #REQUIRED>
    <!ATTLIST date_of_birth year CDATA #REQUIRED>
    <!ELEMENT blood_group (#PCDATA)>
]>