From the text:

He was different from his brothers and sisters. Their hair already betrayed the reddish hue inherited from their mother, the she-wolf; while he alone, in this particular, took after his father. He was the one little grey cub of the litter. He had bred true to the straight wolf-stock—in fact, he had bred true to old One Eye himself, physically, with but a single exception, and that was he had two eyes to his father’s one.