Display RequiredField Validator after PostBack on Button Click in ASP.Net

Hi EveryOne

When i click the signup button, required validatation appears. when i choose dropdownlist before entering data in textbox the required validations disappear.

So how i should prevent disappearing keep in mind i need the autopostback = true in order to obtain the value of dropdownlist on button click

please excuse my poor english

    <form id="form1" runat="server">
        <div class="form-group">
            <label class="control-label">
                Name</label>

            <asp:TextBox ID="TextBoxName" runat="server" CssClass="form-control"
                Font-Size="Large"></asp:TextBox>
            <asp:RequiredFieldValidator ID="RequiredFieldValidator8" ControlToValidate="TextBoxName" runat="server" ErrorMessage="*" ValidationGroup="S" ForeColor="Red"></asp:RequiredFieldValidator>
        </div>

        <div class="form-group">
            <label class="control-label">
                Gender</label>
            <asp:DropDownList ID="DDLforGender" runat="server" class="form-title form-control"
                Font-Size="Large" AutoPostBack="True" ForeColor="#888888">
            </asp:DropDownList>
            <asp:RequiredFieldValidator ID="RequiredFieldValidator2" ControlToValidate="DDLforGender" InitialValue="0" runat="server" ValidationGroup="S" ErrorMessage="*" ForeColor="Red"></asp:RequiredFieldValidator>
        </div>        
        <asp:Button ID="Button6" runat="server" CssClass="btn btn-success large-text btn-lg" OnClick="Button6_Click" Text="SignUp" UseSubmitBehavior="false" ValidationGroup="S" />
    </form>