Notify ObservableCollection when property changes

Question

I have the following classes.

Class Contract
{
   public double Bid { get; set; }
   ...
}

Class Leg
{
   public Contract con {get; set; }
   ...
}

class Combo
{
   public Leg L1 { get; set;}
   public Leg L2 { get; set;}
   ....
}

ObservableCollection<Combo> listOfCombos = new ObservableCollection<Combo>();
Combo cmbo = new Combo();
listOfCombos.Add(cmbo);

//datagrid bound to ObservableCollection
<DataGrid x:Name="dgCombos" ItemsSource="{Binding listOfCombos,Mode=OneWay}"

How do I notify the listOfCombos when a Bid property of a Contract changes. I also want the change to propagate to the datagrid. I know how to notify ObservableCollection when a property of T (the only class) changes. Im confused on how to notify, when theres more than one Class. ie. Combo > Leg > Contract. - Thanks