layout

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="info.rekayasa.radiobutton.MainActivity" >

    <RadioGroup
        android:id="@+id/rgJenis"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        >
        <RadioButton
            android:id="@+id/rbJenisLk"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Laki-laki"
            android:checked="true"
            />
        <RadioButton
            android:id="@+id/rbJenisPr"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Perempuan"
            />
    </RadioGroup>
    <Button
        android:id="@+id/btnCek"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Cek"
        android:layout_below="@+id/rgJenis"
        android:layout_marginTop="20dp"
        />

</RelativeLayout>