radio button group : event is not firing when post back is true and when checked is set for on radio button as default value

See more:

.NET3.5

C#

ASP.NET

.NET

i have a page

<%@ Page Title="" Language="C#" MasterPageFile="~/UserArea/MainMaster.master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <p>
        <br />
        <asp:RadioButton ID="RadioButton1" runat="server" AutoPostBack="True"
            Checked="True" GroupName="a" oncheckedchanged="RadioButton1_CheckedChanged" />
        <asp:RadioButton ID="RadioButton2" runat="server" AutoPostBack="True"
            GroupName="a" oncheckedchanged="RadioButton2_CheckedChanged" />
        <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
    </p>
    <p>
        &nbsp;</p>
    <p>
    </p>
</asp:Content>

and CS page

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
      //
    }
    protected void RadioButton1_CheckedChanged(object sender, EventArgs e)
    {
//code block
    }
    protected void RadioButton2_CheckedChanged(object sender, EventArgs e)
    {
//code block
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        RadioButton2.Checked = true;
    }
}

its my requirement that radio button 1 is checked during page load and when button pressed it change to unchecked and radio button 2 is checked.my problem is after pressing button checked change event is not firing..plzz help.........