Javascript validation for asp.net radiobutton

HTML:

<asp:RadioButton Text="Male" runat="server" ID="rbMale" GroupName="m" />

<asp:RadioButton Text="Female" runat="server" ID="rbFemale" GroupName="m" />

<br />

 <asp:Button Text="Submit" ID="btn" runat="server" OnClientClick="return Validate()"

 />

<br />

 <span id="spn"></span>

Rendering of RadioButtons:

<input id="MainContent_rbMale" type="radio" name="ctl00$MainContent$m" value="rbMale">

<label for="MainContent_rbMale">Male</label>

<input id="MainContent_rbFemale" type="radio" name="ctl00$MainContent$m" value="rbFemale">

<label for="MainContent_rbFemale">Female</label>

Javascript:

<script type="text/javascript">

        function Validate() {

            var rbM = document.getElementById("<%=rbMale.ClientID %>");

            var rbF = document.getElementById("<%=rbFemale.ClientID %>");

           

            if (rbM.checked) {

                document.getElementById("spn").innerHTML = "You have selected: " + rbM.nextSibling.innerHTML;

                return false;

            }

            else if (rbF.checked) {

                document.getElementById("spn").innerHTML = "You have selected: " + rbF.nextSibling.innerHTML;

                return false;

            }

            else {

                alert("Please select Gender!");

                return false;

            }

            return true;

        }

    </script>