Ion-radio checked not showing

checked ion-radio does not show with checked="true"
Not sure why. Any help?

Thanks & much appreciated!

<ion-list radio-group [(ngModel)]="scheduleSearch.week" name="week" required>
      <ion-list-header>Week to search:</ion-list-header>
      <ion-item>
        <ion-label>This Week</ion-label>
        <ion-radio value="thisWeek" color="danger" checked="true"></ion-radio>
      </ion-item>
      <ion-item>
        <ion-label>Next Week</ion-label>
        <ion-radio value="nextWeek" color="danger"></ion-radio>
      </ion-item>
</ion-list>