c# - Relating two comboboxes in WPF

Question

I have a WPF Application with two ComboBoxes.
When I select the first one the items related to the first combobox will be populated on the second one

Here is my select Property

   public string SelectedApplication
    {
        set
        {
            if (_selectedApplication == value) return;
            this._selectedApplication = value;

            InitializeTransactionTypes();


        }
        get
        {
            return this._selectedApplication;
        }

    }

here i am checking a matching id between the two comboboxes to populate the second control items.

    ObservableCollection<TransactionTypeViewModel> _transTypeObsList = new ObservableCollection<TransactionTypeViewModel>();
       private void InitializeTransactionTypes()
    {
        if (_selectedApplication != null)
        {
            if (transactionTypes != null)
            {
                transactionTypes.Clear();
            }

            var getAppCode =
                ApplicationVModel.GetAllApplications().FirstOrDefault(apps => apps.Name == _selectedApplication);

            var transTypeList = TransactionTypeVModel.GetAllTransactionTypes()
                                                     .Where(t => getAppCode != null && t.Id == getAppCode.Id)
                                                     .ToList();

            _transTypeObsList = new ObservableCollection<TransactionTypeViewModel>(transTypeList);
        }
        this.transactionTypes = _transTypeObsList;

        NotifyPropertyChanged("TransactionTypes");
    }

Lets say I select first combobox has {A,B,C,D} ...and the second combobox has {A'1,A'2,A'3}, when I select item from first Combobox the second combobo keeps populating items. I wanted to show only {A'1 for A} {B'1 for B} ...etc but now what it does is {A'1 A'1 A'1 ..... for A} {B'1 B'1 B'1 ....for B} for every select.

EDIT:

List of VM mapped from List of Model

       public List<TransactionTypeViewModel> GetAllViewModelTransTypes()
    {
        TransactionTypeViewModels =   TransactionTypeModel.GetAllTransactionTypes().Select(transType => new TransactionTypeViewModel
        {
            Id = transType.Id,
            Name = transType.Name,
            RequestMessageType = transType.RequestMessageType,
            ResponseMessageTypes = transType.ResponseMessageTypes,
            WsMethodName = transType.WSMethodName,
            WsRequestXmlType = transType.WSRequestXmlType,
            WsResponseXmlType = transType.WSResponseXmlType,
            XsdFile = transType.XsdFile,

        })
    .ToList();
        return TransactionTypeViewModels;
    }

List of Observable VM from List of VM

     public ObservableCollection<TransactionTypeViewModel> GetAllTransactionTypes()
    {
        TransactionTypeViewModelList = this.GetAllViewModelTransTypes();
        ObservableTransactionType = new ObservableCollection<TransactionTypeViewModel>(TransactionTypeViewModelList);

        return ObservableTransactionType;
    }

EDIT:

  private void InitializeTransactionTypes()
    {
        if (_selectedApplication != null)
        {

            var getAppCode =
                ApplicationVModel.GetAllApplications()
                                 .FirstOrDefault(apps => apps.Name == _selectedApplication);

            var transTypeList = TransactionTypeVModel.GetAllTransactionTypes()
                                                     .Where(t => getAppCode != null && t.Id == getAppCode.Id);

            transactionTypes = new ObservableCollection<TransactionTypeViewModel>(transTypeList); 

           NotifyPropertyChanged("TransactionTypes");
        }
    }