WPF cascaded combo

I am using LINQ to SQL and MVVM pattern in my application where i am retrieving my data by the following query:

internal ObservableCollection<INVCategory> GetCategoryList()
{
     DataLoadOptions dataLoadOptions = new DataLoadOptions();
     dataLoadOptions.LoadWith<INVCategory>(t => t.INVSubCategories);
     this.Context.LoadOptions = dataLoadOptions;

     var categories = from category in this.Context.INVCategories
                      orderby category.CatgeoryId descending
                      select category;
     return new ObservableCollection<INVCategory>(categories.ToList());
}

And my XAML code for parent(Category) combo is:

<ComboBox Grid.Column="1" Grid.Row="0" HorizontalAlignment="Stretch"                           VerticalAlignment="Stretch" Margin="0,2,10,2" TabIndex="0" x:Name="categoryComboBox"      
ItemsSource="{Binding CategoryList}" IsEditable="True" DisplayMemberPath="CategoryName"  SelectedValuePath="CatgeoryId" SelectedItem="{Binding CategoryList, Mode=TwoWay}" SelectedValue="{Binding Path=CurrentEntity.CategoryId, Mode=TwoWay}" >
</ComboBox>

for child(Subcategory) combo i am using:

<ComboBox Grid.Column="1" Grid.Row="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="0,2,10,2" TabIndex="1"
ItemsSource="{Binding SelectedItem, ElementName=categoryComboBox, Mode=OneWay}" 
DisplayMemberPath="SubCategoryName" SelectedValuePath="SubCategoryId"
SelectedItem="{Binding INVSubCategories, Mode=TwoWay}" >
</ComboBox>

But my child combo items is not populated during form loading as well as parent combo's selection changed though my parent combo items are popul开发者_开发知识库ated. i can't figure out why my child combo is not functioning based on parent combo's selected item ,please help me.