checkbox单选(多个checkbox时只能选中一个)

<table id="tab" cellpadding="0" cellspacing="0" border="0" width="100%">
                                    <tr height="25px">
                                        <td align="right" width="40%">
                                            <asp:Label ID="Label1" runat="server" Text="模板描述："></asp:Label>
                                        </td>
                                        <td>
                                            <asp:TextBox ID="txtTempletText" runat="server" ReadOnly="true"></asp:TextBox>
                                        </td>
                                    </tr>
                                    <tr height="25px">
                                        <td align="right">
                                            <input type="checkbox" id="chkPer" value="1" runat="server" onclick="selchk(this)" />本人
                                        </td>
                                        <td>
                                            &nbsp;&nbsp;&nbsp;&nbsp;
                                            <input type="checkbox" id="chkAll" value="3" runat="server" onclick="selchk(this)" />本院(共用)
                                        </td>
                                    </tr>
                                    <tr height="25px">
                                        <td>
                                        </td>
                                        <td>
                                            &nbsp;&nbsp;&nbsp;&nbsp;
                                            <input type="checkbox" id="chkDept" value="2" runat="server" onclick="selchk(this)" />本科室(共用)
                                        </td>
                                    </tr>
                                </table>

function selchk(chk)  //chk 为 checkbox控件，   前台每个checkbox控件都需加上这方法
        {
            for(var i=0;i<form1.elements.length;i++)
            {
                var e = form1.elements[i];
                if(e.type!=null && e.type=="checkbox" && e.parentElement.parentElement.parentElement.parentElement.id=="tab") //表格里的所有checkbox是否全部选种
                {
                    if(e.checked)
                    {
                        e.checked = false;
                        chk.checked = true;
                    }
                }
            }
        }